Add support for JSON files containing a single tiddler

At the moment, we support JSON files containing an array of tiddlers.
With this change the core will import files containing a single
tiddler. Also adding templates for saving individual tiddlers in JSON
format

core/modules/macros/jsontiddler.js

@@ -0,0 +1,40 @@
+/*\
+title: $:/core/modules/macros/jsontiddler.js
+type: application/javascript
+module-type: macro
+
+Macro to output a single tiddler to JSON
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Information about this macro
+*/
+
+exports.name = "jsontiddler";
+
+exports.params = [
+	{name: "title"}
+];
+
+/*
+Run the macro
+*/
+exports.run = function(title) {
+	title = title || this.getVariable("currentTiddler");
+	var tiddler = !!title && this.wiki.getTiddler(title),
+		fields = new Object();
+	if(tiddler) {
+		for(var field in tiddler.fields) {
+			fields[field] = tiddler.getFieldString(field);
+		}
+	}
+	return JSON.stringify(fields,null,$tw.config.preferences.jsonSpaces);
+};
+
+})();