Add 'get' filter operator

Prompted by @sukima’s work on #586

core/modules/filters/get.js

@@ -0,0 +1,31 @@
+/*\
+title: $:/core/modules/filters/get.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator for replacing tiddler titles by the value of the field specified in the operand.
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.get = function(source,operator,options) {
+	var results = [];
+	source(function(tiddler,title) {
+		if(tiddler) {
+			var value = tiddler.getFieldString(operator.operand);
+			if(value) {
+				results.push(value);
+			}
+		}
+	});
+	return results;
+};
+
+})();

editions/tw5.com/tiddlers/filters/FilterOperator get.tid

@@ -0,0 +1,12 @@
+created: 20140426183123179
+modified: 20140426183123179
+tags: filters
+title: FilterOperator: get
+type: text/vnd.tiddlywiki
+
+The ''get'' filter operator replaces the titles of the tiddlers in the current list with the values of the field specified by the operand.
+
+For example:
+
+|!Filter String |!Description |
+|`[all[current]get[draft.of]]` |If the current tiddler is a draft, returns the original tiddler, otherwise returns an empty list |