feat(styling): keep Nuxt style when css=false (#218)

* fix: improve inline style detection

* feat: disable amp specific styles

This is useful on pure amp project. With this developer can use SFC styling.

* docs: update docs

docs/api/options.md

@@ -6,7 +6,7 @@ General options of amp module
 | Option  | Default | Valid values | Description |
 | ------  | ------- | ------------ | ----------- |
 | cdnBase | `https://cdn.ampproject.org/v0/` | Any String | A CDN Domain to load AMP elements scripts |
-| css | `undefined` | Path to CSS style | Custom styles for AMP pages, eg. `~/assets/style/amp-custom.css` |
+| css     | `undefined` | `false` or Path to CSS style | Custom styles for AMP pages, eg. `~/assets/style/amp-custom.css`. By setting this option to `false`, the module will not remove inline styles, instead it will try to merge all inline styles into a single style tag.|
 |  tags   | `{}`    | Object of tag options  | Define new tags or modify current tags, for instance if you want to use `amp-mustache` version 0.1, tags value must be `{ 'amp-mustache': { version: '0.1' } }` |
 | origin  | `` | Any String | Main domain of website. Using this AMP modules tries to add missing canonical link for pages. |
 | mode    | `hybrid` | `only\|hybrid\|false` | Default behaviour of amp module. (`only` all pages serve in AMP mode by default, `hybrid` pages serves in both normal and AMP mode, `false` pages does not serve AMP by default ) |

lib/module.js

@@ -159,11 +159,25 @@ function registerRendererHook (options) {
 
     params.HEAD = params.HEAD
       .replace(scriptPattern, v => (v.includes('custom-element') || v.includes('application/ld+json')) ? v : '')
+
+    if (options.css !== false) {
+      /**
+       * Remove inline styles
+       */
+      params.HEAD = params.HEAD
+        .replace(/<style[^>]*>[.\S\s]*?<\/style>/g, '')
+    } else {
       /**
-       * Remove external styles
+       * Merge inline styles
        */
-      .replace(/<style[^>]*>.*?<\/style>/g, '')
+      params.HEAD = params.HEAD.replace(/<\/style>\S*<style [^>]*>/g, '')
+        .replace(/<style/, '<style amp-custom')
+        .replace('@charset "UTF-8";', '')
+    }
 
+    /**
+     * Remove external styles
+     */
     params.HEAD = removeExternalStyles(params.HEAD)
 
     params.HEAD += AMPBoilerplate