Add support for autosave

Causes the wiki to be autosaved whenever clicking “done” after editing
a tiddler. Only works with savers that support autosave. We should
probably make autosave configurable

core/modules/savers/download.js

@@ -46,7 +46,8 @@ Information about this saver
 */
 DownloadSaver.prototype.info = {
 	name: "download",
-	priority: 100
+	priority: 100,
+	capabilities: ["save", "download"]
 };
 
 /*