First pass at KaTeX plugin

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

plugins/tiddlywiki/katex/wrapper.js

@@ -0,0 +1,69 @@
+/*\
+title: $:/plugins/tiddlywiki/katex/wrapper.js
+type: application/javascript
+module-type: widget
+
+Wrapper for `katex.min.js` that provides a `<$latex>` widget. It is also available under the alias `<$katex>`
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var katex = require("$:/plugins/tiddlywiki/katex/katex.min.js"),
+	Widget = require("$:/core/modules/widgets/widget.js").widget;
+
+var KaTeXWidget = function(parseTreeNode,options) {
+	this.initialise(parseTreeNode,options);
+};
+
+/*
+Inherit from the base widget class
+*/
+KaTeXWidget.prototype = new Widget();
+
+/*
+Render this widget into the DOM
+*/
+KaTeXWidget.prototype.render = function(parent,nextSibling) {
+	// Housekeeping
+	this.parentDomNode = parent;
+	this.computeAttributes();
+	this.execute();
+	// Get the source text
+	var text = this.getAttribute("text",this.parseTreeNode.text || "");
+	// Render it into a span
+	var span = this.document.createElement("span");
+	katex.render(text,span);
+	// Insert it into the DOM
+	parent.insertBefore(span,nextSibling);
+	this.domNodes.push(span);
+};
+
+/*
+Compute the internal state of the widget
+*/
+KaTeXWidget.prototype.execute = function() {
+	// Nothing to do for a katex widget
+};
+
+/*
+Selectively refreshes the widget if needed. Returns true if the widget or any of its children needed re-rendering
+*/
+KaTeXWidget.prototype.refresh = function(changedTiddlers) {
+	var changedAttributes = this.computeAttributes();
+	if(changedAttributes.text) {
+		this.refreshSelf();
+		return true;
+	} else {
+		return false;	
+	}
+};
+
+exports.latex = KaTeXWidget;
+exports.katex = KaTeXWidget;
+
+})();
+