Add new "count" filter operator

See discussion here:
https://groups.google.com/d/msgid/tiddlywiki/fe51b7f5-5369-493b-82e5-94c
18e863fe0%40googlegroups.com?utm_medium=email&utm_source=footer

core/modules/filters/count.js

@@ -0,0 +1,26 @@
+/*\
+title: $:/core/modules/filters/count.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator returning the number of entries in the current list.
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.count = function(source,operator,options) {
+	var count = 0;
+	source(function(tiddler,title) {
+		count++;
+	});
+	return [count + ""];
+};
+
+})();

editions/tw5.com/tiddlers/filters/count_Operator.tid

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+caption: count
+created: 20170208200553517
+modified: 20170208200900081
+op-input: a [[selection of titles|Title Selection]]
+op-output: a new list containing the number of items in the input list as a decimal string
+op-purpose: count the number of entries in a list
+tags: [[Filter Operators]] [[Group Operators]]
+title: count Operator
+type: text/vnd.tiddlywiki
+
+The number of the entries in the input list is counted and a decimal representation returned as a single string.
+
+<<.operator-examples "count">>

editions/tw5.com/tiddlers/filters/examples/count_Operator_(Examples).tid

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+created: 20170208200912602
+modified: 20170208201020488
+tags: [[each Operator]] [[Operator Examples]]
+title: count Operator (Examples)
+type: text/vnd.tiddlywiki
+
+<<.operator-example 1 "[tag[HelloThere]count[]]">>