Text-slicer: Add list-children filter operator

Again, I needed this for the day job. @felixhayashi I think you
submitted a pull request for something similar, would this version meet
your needs?

plugins/tiddlywiki/text-slicer/modules/filters/list-children.js

@@ -0,0 +1,38 @@
+/*\
+title: $:/core/modules/filters/list-children.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator returning all the descendents of a tiddler listed in the "list" field
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports["list-children"] = function(source,operator,options) {
+	var children = {},
+		processTiddler = function(title) {
+			var tiddler = options.wiki.getTiddler(title);
+			if(tiddler && !$tw.utils.hop(children,title)) {
+				children[title] = true;
+				var list = options.wiki.getTiddlerList(title,operator.operand);
+				list.forEach(function(listItem) {
+					if(!$tw.utils.hop(children,listItem)) {
+						processTiddler(listItem);
+					}
+				});
+			}
+		};
+	source(function(tiddler,title) {
+		processTiddler(title);
+	});
+	return Object.keys(children);
+};
+
+})();