Introduce refactored wiki parser and renderer

This is a half-way through a big refactoring of the parsing and
rendering infrastructure. The main change is to separate the parse and
render trees, which makes the code a lot cleaner. The new parser isn't
yet functional enough to replace the existing parser so for the moment
you have to manually invoke it with `$tw.testNewParser()` in your
browser console. I really ought to use branches for this kind of
thing...

core/modules/widgets/view/viewers/date.js

@@ -0,0 +1,36 @@
+/*\
+title: $:/core/modules/widgets/view/viewers/date.js
+type: application/javascript
+module-type: newfieldviewer
+
+A viewer for viewing tiddler fields as a date
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var DateViewer = function(viewWidget,tiddler,field,value) {
+	this.viewWidget = viewWidget;
+	this.tiddler = tiddler;
+	this.field = field;
+	this.value = value;
+};
+
+DateViewer.prototype.render = function() {
+	var template = this.viewWidget.renderer.getAttribute("template","DD MMM YYYY"),
+		value = "";
+	if(this.value !== undefined) {
+		value = $tw.utils.formatDateString(this.value,template);
+	}
+	return this.viewWidget.renderer.renderTree.createRenderers(this.viewWidget.renderer.renderContext,[{
+		type: "text",
+		text: value
+	}]);
+};
+
+exports.date = DateViewer;
+
+})();