First pass at an --editions command

Also added a “description” field for wiki folders.

Right now there’s too many editions listed. I think we should by
default only list editions whose `tiddlywiki.info` file has
`showInListings` set to `true`, and have an `--editions all` command
that lists everything.

@pmario please could you check that the editions in your
`TIDDLYWIKI_EDITION_PATH` are correctly listed?

core/modules/commands/editions.js

@@ -0,0 +1,62 @@
+/*\
+title: $:/core/modules/commands/editions.js
+type: application/javascript
+module-type: command
+
+Command to list the available editions
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+exports.info = {
+	name: "editions",
+	synchronous: true
+};
+
+var Command = function(params,commander) {
+	this.params = params;
+	this.commander = commander;
+};
+
+Command.prototype.execute = function() {
+	var fs = require("fs"),
+		path = require("path"),
+		self = this;
+	// Enumerate the edition paths
+	var editionPaths = $tw.getLibraryItemSearchPaths($tw.config.editionsPath,$tw.config.editionsEnvVar),
+		editions = {};
+	for(var editionIndex=0; editionIndex<editionPaths.length; editionIndex++) {
+		var editionPath = editionPaths[editionIndex];
+		// Enumerate the folders
+		var entries = fs.readdirSync(editionPath);
+		for(var entryIndex=0; entryIndex<entries.length; entryIndex++) {
+			var entry = entries[entryIndex];
+			// Check if directories have a valid tiddlywiki.info
+			if(!editions[entry] && $tw.utils.isDirectory(path.resolve(editionPath,entry))) {
+				var info;
+				try {
+					info = JSON.parse(fs.readFileSync(path.resolve(editionPath,entry,"tiddlywiki.info"),"utf8"));
+				} catch(ex) {
+				}
+				if(info) {
+					editions[entry] = info.description || "";
+				}
+			}
+		}
+	}
+	// Output the list
+	this.commander.streams.output.write("Available editions:\n\n");
+	$tw.utils.each(editions,function(description,name) {
+		self.commander.streams.output.write("    " + name + ": " + description + "\n");
+	});
+	this.commander.streams.output.write("\n");
+	return null;
+};
+
+exports.Command = Command;
+
+})();