Add unusedtitle macro (#3880)

* Add uniquetitle macro

This macro is a wrapper on the $tw.wiki.generateNewTitle function to return a unique name, it has been requested multiple times in the forums.

* Add documentation tiddler for uniquetitle macro

* Add uniquetitle macro examples

* Change name from uniquetitle to unusedtitle for clarity

The title isn't unique, just not currently used in the wiki.

* Update uniquetitle to unusedtitle

* Change uniquetitle to unusedtitle

* Fix a typo

core/modules/macros/uniquetitle.js

@@ -0,0 +1,34 @@
+/*\
+title: $:/core/modules/macros/unusedtitle.js
+type: application/javascript
+module-type: macro
+Macro to return a new title that is unused in the wiki. It can be given a name as a base.
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Information about this macro
+*/
+
+exports.name = "unusedtitle";
+
+exports.params = [
+	{name: "baseName"},
+	{name: "options"}
+];
+
+/*
+Run the macro
+*/
+exports.run = function(baseName, options) {
+	if(!baseName) {
+		baseName = $tw.language.getString("DefaultNewTiddlerTitle");
+	}
+	return this.wiki.generateNewTitle(baseName, options);
+};
+
+})();