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

_src/HighlightLines.js

@@ -0,0 +1,33 @@
+class HighlightLines {
+	constructor(rangeStr) {
+		this.highlights = this.convertRangeToHash(rangeStr);
+	}
+
+	convertRangeToHash(rangeStr) {
+		let hash = {};
+		if( !rangeStr ) {
+			return hash;
+		}
+
+		let ranges = rangeStr.split(",").map(function(range) {
+			return range.trim();
+		});
+
+		for(let range of ranges) {
+			let startFinish = range.split('-');
+			let start = parseInt(startFinish[0], 10);
+			let end = parseInt(startFinish[1] || start, 10);
+
+			for( let j = start, k = end; j<=k; j++ ) {
+				hash[j] = true;
+			}
+		}
+		return hash;
+	}
+
+	isHighlighted(lineNumber) {
+		return !!this.highlights[lineNumber]
+	}
+}
+
+module.exports = HighlightLines;