Clean up Previewers' page (#1630)

This PR makes some changes to the [Previewers plugin's page](https://prismjs.com/plugins/previewers/).

1. Format the code examples to look nicer.
2. Fix the example of the 'Disabling a previewer' section.
3. Highlight API snippets as JavaScript.

Author: RunDevelopment

plugins/previewers/index.html

@@ -43,19 +43,19 @@ <h1>Examples</h1>
 
 	<h2>CSS</h2>
 	<pre class="language-css"><code>.example-gradient {
-	background: -moz-linear-gradient(left,  #cb60b3 0%, #c146a1 50%, #a80077 51%, #db36a4 100%); /* FF3.6+ */
-	background: -webkit-linear-gradient(left,  #cb60b3 0%,#c146a1 50%,#a80077 51%,#db36a4 100%); /* Chrome10+,Safari5.1+ */
-	background: -o-linear-gradient(left,  #cb60b3 0%,#c146a1 50%,#a80077 51%,#db36a4 100%); /* Opera 11.10+ */
-	background: -ms-linear-gradient(left,  #cb60b3 0%,#c146a1 50%,#a80077 51%,#db36a4 100%); /* IE10+ */
-	background: linear-gradient(to right,  #cb60b3 0%,#c146a1 50%,#a80077 51%,#db36a4 100%); /* W3C */
+	background: -webkit-linear-gradient(left,     #cb60b3 0%, #c146a1 50%, #a80077 51%, #db36a4 100%); /* Chrome10+, Safari5.1+ */
+	background:    -moz-linear-gradient(left,     #cb60b3 0%, #c146a1 50%, #a80077 51%, #db36a4 100%); /* FF3.6+ */
+	background:     -ms-linear-gradient(left,     #cb60b3 0%, #c146a1 50%, #a80077 51%, #db36a4 100%); /* IE10+ */
+	background:      -o-linear-gradient(left,     #cb60b3 0%, #c146a1 50%, #a80077 51%, #db36a4 100%); /* Opera 11.10+ */
+	background:         linear-gradient(to right, #cb60b3 0%, #c146a1 50%, #a80077 51%, #db36a4 100%); /* W3C */
 }
 .example-angle {
 	transform: rotate(10deg);
 }
 .example-color {
 	color: rgba(255, 0, 0, 0.2);
 	background: purple;
-	border: 1px solid hsl(100,70%,40%);
+	border: 1px solid hsl(100, 70%, 40%);
 }
 .example-easing {
 	transition-timing-function: linear;
@@ -69,13 +69,13 @@ <h2>Markup attributes</h2>
 &lt;tr style="background: lightblue;"></code></pre>
 
 	<h2>Less</h2>
-	<pre class="language-less"><code>@gradient: linear-gradient(135deg,  #9dd53a 0%,#a1d54f 50%,#80c217 51%,#7cbc0a 100%);
+	<pre class="language-less"><code>@gradient: linear-gradient(135deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%);
 .example-gradient {
-	background: -moz-linear-gradient(-45deg,  #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%); /* FF3.6+ */
-	background: -webkit-linear-gradient(-45deg,  #9dd53a 0%,#a1d54f 50%,#80c217 51%,#7cbc0a 100%); /* Chrome10+,Safari5.1+ */
-	background: -o-linear-gradient(-45deg,  #9dd53a 0%,#a1d54f 50%,#80c217 51%,#7cbc0a 100%); /* Opera 11.10+ */
-	background: -ms-linear-gradient(-45deg,  #9dd53a 0%,#a1d54f 50%,#80c217 51%,#7cbc0a 100%); /* IE10+ */
-	background: linear-gradient(135deg,  #9dd53a 0%,#a1d54f 50%,#80c217 51%,#7cbc0a 100%); /* W3C */
+	background: -webkit-linear-gradient(-45deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%); /* Chrome10+, Safari5.1+ */
+	background:    -moz-linear-gradient(-45deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%); /* FF3.6+ */
+	background:     -ms-linear-gradient(-45deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%); /* IE10+ */
+	background:      -o-linear-gradient(-45deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%); /* Opera 11.10+ */
+	background:         linear-gradient(135deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%); /* W3C */
 }
 @angle: 3rad;
 .example-angle {
@@ -85,7 +85,7 @@ <h2>Less</h2>
 .example-color {
 	color: hsla(102, 53%, 42%, 0.4);
 }
-@easing: cubic-bezier(0.1,0.3,1,.4);
+@easing: cubic-bezier(0.1, 0.3, 1, .4);
 .example-easing {
 	transition-timing-function: ease;
 }
@@ -95,20 +95,20 @@ <h2>Less</h2>
 }</code></pre>
 
 	<h2>Sass</h2>
-	<pre class="language-sass"><code>$gradient: linear-gradient(135deg,  #9dd53a 0%,#a1d54f 50%,#80c217 51%,#7cbc0a 100%)
+	<pre class="language-sass"><code>$gradient: linear-gradient(135deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%)
 @mixin example-gradient
-	background: -moz-radial-gradient(center, ellipse cover,  #f2f6f8 0%, #d8e1e7 50%, #b5c6d0 51%, #e0eff9 100%)
-	background: radial-gradient(ellipse at center,  #f2f6f8 0%,#d8e1e7 50%,#b5c6d0 51%,#e0eff9 100%)
+	background: -moz-radial-gradient(center, ellipse cover, #f2f6f8 0%, #d8e1e7 50%, #b5c6d0 51%, #e0eff9 100%)
+	background: radial-gradient(ellipse at center, #f2f6f8 0%, #d8e1e7 50%, #b5c6d0 51%, #e0eff9 100%)
 $angle: 380grad
 @mixin example-angle
 	transform: rotate(-120deg)
 .example-angle
 	transform: rotate(18rad)
 $color: blue
 @mixin example-color
-    color: rgba(147, 32, 34, 0.8)
+	color: rgba(147, 32, 34, 0.8)
 .example-color
-    color: pink
+	color: pink
 $easing: ease-out
 .example-easing
 	transition-timing-function: ease-in-out
@@ -120,32 +120,32 @@ <h2>Sass</h2>
 </code></pre>
 
 	<h2>Scss</h2>
-	<pre class="language-scss"><code>$gradient: linear-gradient(135deg,  #9dd53a 0%,#a1d54f 50%,#80c217 51%,#7cbc0a 100%);
+	<pre class="language-scss"><code>$gradient: linear-gradient(135deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%);
 $attr: background;
 .example-gradient {
-    #{$attr}-image: repeating-linear-gradient(10deg, rgba(255,0,0,0), rgba(255,0,0,1) 10px, rgba(255,0,0,0) 20px);
+	#{$attr}-image: repeating-linear-gradient(10deg, rgba(255, 0, 0, 0), rgba(255, 0, 0, 1) 10px, rgba(255, 0, 0, 0) 20px);
 }
 $angle: 1.8turn;
 .example-angle {
 	transform: rotate(-3rad)
 }
 $color: blue;
 .example-color {
-    #{$attr}-color: rgba(255,255,0,0.75);
+	#{$attr}-color: rgba(255, 255, 0, 0.75);
 }
 $easing: linear;
 .example-easing {
-    transition-timing-function: cubic-bezier(0.9,0.1,.2,.4);
+	transition-timing-function: cubic-bezier(0.9, 0.1, .2, .4);
 }
 $time: 1s;
 .example-time {
 	transition-duration: 10s
 }</code></pre>
 
 	<h2>Stylus</h2>
-	<pre class="language-stylus"><code>gradient = linear-gradient(135deg,  #9dd53a 0%,#a1d54f 50%,#80c217 51%,#7cbc0a 100%)
+	<pre class="language-stylus"><code>gradient = linear-gradient(135deg, #9dd53a 0%, #a1d54f 50%, #80c217 51%, #7cbc0a 100%)
 .example-gradient
-	background-image: repeating-radial-gradient(circle, rgba(255,0,0,0), rgba(255,0,0,1) 10px, rgba(255,0,0,0) 20px)
+	background-image: repeating-radial-gradient(circle, rgba(255, 0, 0, 0), rgba(255, 0, 0, 1) 10px, rgba(255, 0, 0, 0) 20px)
 angle = 357deg
 .example-angle
 	transform: rotate(100grad)
@@ -166,15 +166,16 @@ <h1>Disabling a previewer</h1>
 		can be added on a code block or any ancestor. Its value should be a space-separated list of previewers
 		representing the subset.</p>
 	<p>For example:</p>
-	<pre class="language-markup"><code>&lt;pre class="language-css" data-previewers="color">&lt;code>div {
-	/* Only the previewer for colors is enabled */
+	<pre class="language-markup"><code>&lt;pre class="language-css" data-previewers="color time">&lt;code>div {
+	/* Only the previewer for color and time are enabled */
 	color: red;
+	transition-duration: 1s;
 	/* The previewer for angles is not enabled. */
 	transform: rotate(10deg);
 }&lt;/code>&lt;/pre></code></pre>
 	<p>will give the following result:</p>
 	<pre class="language-css" data-previewers="color time"><code>div {
-	/* Only the previewers color and time are enabled */
+	/* Only the previewers for color and time are enabled */
 	color: red;
 	transition-duration: 1s;
 	/* The previewer for angles is not enabled. */
@@ -184,30 +185,30 @@ <h1>Disabling a previewer</h1>
 
 <section>
 	<h1>API</h1>
-	<p>This plugins provides a constructor that can be accessed through <code>Prism.plugins.Previewer</code>.</p>
+	<p>This plugins provides a constructor that can be accessed through <code class="language-javascript">Prism.plugins.Previewer</code>.</p>
 	<p>Once a previewer has been instantiated, an HTML element is appended to the document body.
 		This element will appear when specific tokens are hovered.</p>
 
-	<h2><code>new Prism.plugins.Previewer(type, updater, supportedLanguages)</code></h2>
+	<h2><code class="language-javascript">new Prism.plugins.Previewer(type, updater, supportedLanguages)</code></h2>
 
 	<ul>
 		<li>
-			<p><code>type</code>: the token type this previewer is associated to.
+			<p><code class="language-javascript">type</code>: the token type this previewer is associated to.
 				The previewer will be shown when hovering tokens of this type.</p>
 		</li>
 		<li>
-			<p><code>updater</code>: the function that will be called each time an associated token is hovered.
+			<p><code class="language-javascript">updater</code>: the function that will be called each time an associated token is hovered.
 				This function takes the text content of the token as its only parameter.
-				The previewer HTML element can be accessed through the keyword <code>this</code>.
-				This function must return <code>true</code> for the previewer to be shown.</p>
+				The previewer HTML element can be accessed through the keyword <code class="language-javascript">this</code>.
+				This function must return <code class="language-javascript">true</code> for the previewer to be shown.</p>
 		</li>
 		<li>
-			<p><code>supportedLanguages</code>: an optional array of supported languages.
+			<p><code class="language-javascript">supportedLanguages</code>: an optional array of supported languages.
 				The previewer will be available only for those.
-				Defaults to <code>'*'</code>, which means every languages.</p>
+				Defaults to <code class="language-javascript">'*'</code>, which means every languages.</p>
 		</li>
 		<li>
-			<p><code>initializer</code>: an optional function.
+			<p><code class="language-javascript">initializer</code>: an optional function.
 				This function will be called when the previewer is initialized,
 				right after the HTML element has been appended to the document body.</p>
 		</li>