Introduce setfield command

This new command will let us do the manipulations needed to set up
external image tiddlers

core/modules/commands/setfield.js

@@ -0,0 +1,58 @@
+/*\
+title: $:/core/modules/commands/setfield.js
+type: application/javascript
+module-type: command
+
+Command to modify selected tiddlers to set a field to the text of a template tiddler that has been wikified with the selected tiddler as the current tiddler.
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var widget = require("$:/core/modules/widgets/widget.js");
+
+exports.info = {
+	name: "setfield",
+	synchronous: true
+};
+
+var Command = function(params,commander,callback) {
+	this.params = params;
+	this.commander = commander;
+	this.callback = callback;
+};
+
+Command.prototype.execute = function() {
+	if(this.params.length < 4) {
+		return "Missing parameters";
+	}
+	var self = this,
+		wiki = this.commander.wiki,
+		filter = this.params[0],
+		fieldname = this.params[1] || "text",
+		templatetitle = this.params[2],
+		rendertype = this.params[3] || "text/plain",
+		tiddlers = wiki.filterTiddlers(filter);
+	$tw.utils.each(tiddlers,function(title) {
+		var parser = wiki.parseTiddler(templatetitle),
+			newFields = {},
+			tiddler = wiki.getTiddler(title);
+		if(parser) {
+			var widgetNode = wiki.makeWidget(parser,{variables: {currentTiddler: title}});
+			var container = $tw.fakeDocument.createElement("div");
+			widgetNode.render(container,null);
+			newFields[fieldname] = rendertype === "text/html" ? container.innerHTML : container.textContent;
+		} else {
+			newFields[fieldname] = undefined;
+		}
+		wiki.addTiddler(new $tw.Tiddler(tiddler,newFields));
+	});
+	return null;
+};
+
+exports.Command = Command;
+
+})();