Introduce function operator for calling functions

Can invoke any functions, not just those start with a period. And can pass zero parameters (in contrast when invoked as a custom filter operator there's no way to omit the first parameter).

core/modules/filters/function.js

@@ -0,0 +1,45 @@
+/*\
+title: $:/core/modules/filters/function.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator returning those input titles that are returned from a function
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.function = function(source,operator,options) {
+	var functionName = operator.operands[0],
+		customDefinition = options.widget && options.widget.getVariableInfo && options.widget.getVariableInfo(functionName);
+	if(customDefinition && customDefinition.srcVariable && customDefinition.srcVariable.isFunctionDefinition) {
+		var variables = Object.create(null);
+		// Go through each of the defined parameters, and make a variable with the value of the corresponding operand
+		$tw.utils.each(customDefinition.srcVariable.params,function(param,index) {
+			var value = operator.operands[1 + index]; // Skip over the first operand that gives the function name
+			variables[param.name] = value === undefined ? param["default"] || "" : value;
+		});
+		var list = options.wiki.filterTiddlers(customDefinition.srcVariable.value,options.widget.makeFakeWidgetWithVariables(variables),source);
+		if(operator.prefix === "!") {
+			var results = [];
+			source(function(tiddler,title) {
+				if(list.indexOf(title) === -1) {
+					results.push(title);
+				}
+			});
+			return results;
+		} else {
+			return list;
+		}
+	}
+	// Return an empty list if the function wasn't found
+	return [];
+};
+
+})();

editions/test/tiddlers/tests/data/custom-operators/Parameterised.tid

@@ -13,8 +13,12 @@ title: Output
 <$text text={{{ [[123].multiplybysomething[]] }}}/>
 -
 <$text text={{{ [[123].multiplybysomething[x],[4]] }}}/>
+|
+<$text text={{{ [[123]function[.multiplybysomething]] }}}/>
+-
+<$text text={{{ [[123]function[.multiplybysomething],[x],[4]] }}}/>
 
 +
 title: ExpectedResult
 
-<p>492-984</p>
+<p>492-984|492-984</p>

editions/test/tiddlers/tests/data/custom-operators/Simple.tid

@@ -5,12 +5,17 @@ tags: [[$:/tags/wiki-test-spec]]
 
 title: Output
 
+\whitespace trim
+
 \function .multiplybytwo()
 [multiply[2]]
 \end
 
 <$text text={{{ [[123].multiplybytwo[]] }}}/>
+|
+<$text text={{{ [[123]function[.multiplybytwo]] }}}/>
+
 +
 title: ExpectedResult
 
-<p>246</p>
+<p>246|246</p>