Refactors Liquid syntax highlighters to add line highlights.

Usage (ranges are space separated):

{% highlight js 1,4-6 %}
One range
Adds `highlight-line-active` to lines 1,4,5,6

{% highlight js 3-4 -1 %}
Two ranges (add/remove), remove is N/A
Adds `highlight-line-add` to lines 3,4

{% highlight js -1 3-4 %}
Two ranges (add/remove), add is N/A
Adds `highlight-line-remove` to lines 3,4

{% highlight js 3-4 5,8-10 %}
Two ranges, both are used
Adds `highlight-line-add` to lines 3-4
Adds `highlight-line-remove` to lines 5,8,9,10

css/prism-base16-monokai.dark.css

@@ -17,7 +17,7 @@ code[class*="language-"], pre[class*="language-"] {
 	color: #f8f8f2;
 }
 pre[class*="language-"] {
-	padding: 1em;
+	padding: 1.5em 0;
 	margin: .5em 0;
 	overflow: auto;
 }