First pass at KaTeX plugin

Finally fixes #458 thanks to the KaTeX plugin from Khan Academy

plugins/tiddlywiki/katex/latex-parser.js

@@ -0,0 +1,61 @@
+/*\
+title: $:/plugins/tiddlywiki/katex/latex-parser.js
+type: application/javascript
+module-type: wikirule
+
+Wiki text inline rule for LaTeX. For example:
+
+```
+	$$latex-goes-here$$
+```
+
+This wikiparser can be modified using the rules eg:
+
+```
+\rules except latex-parser 
+\rules only latex-parser 
+```
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+exports.name = "latex-parser";
+exports.types = {inline: true};
+
+exports.init = function(parser) {
+	this.parser = parser;
+	// Regexp to match
+	this.matchRegExp = /\$\$(?!\$)/mg;
+};
+
+exports.parse = function() {
+	// Move past the match
+	this.parser.pos = this.matchRegExp.lastIndex;
+	var reEnd = /\$\$/mg;
+	// Look for the end marker
+	reEnd.lastIndex = this.parser.pos;
+	var match = reEnd.exec(this.parser.source),
+		text;
+	// Process the text
+	if(match) {
+		text = this.parser.source.substring(this.parser.pos,match.index);
+		this.parser.pos = match.index + match[0].length;
+	} else {
+		text = this.parser.source.substr(this.parser.pos);
+		this.parser.pos = this.parser.sourceLength;
+	}
+	return [{
+		type: "latex",
+		attributes: {
+			text: {
+				type: "text",
+				value: text
+			}}
+	}];
+};
+
+})();