Add the ability to replace declaration values

[dan-gamble]

This adds the replaceValues option to the plugin

replaceValues

Type boolean
Default: false

If this is true then this plugin will look through your declaration values for the placeholder symbol and replace them with the desired selector.

i.e.

.foo {
    background-color: red;

    &:hover {
        background-color: blue;

        &::before {
            content: '^&';
        }
    }
}

.foo {
  background-color: red;
}

.foo:hover {
 background-color: blue;
}

.foo:hover::before {
 content: '.foo'
}

[dan-gamble]

Quick note, i still need to write a test for this, missed that :)

[toomuchdesign]

Merged with #3.