Improve logic for initialising reading node tiddlers

We want fine control of whether tiddlers are read from the DOM or the
file system, without ganging it to whether we’re on node vs. browser.

boot/boot.js

@@ -1411,7 +1411,7 @@ $tw.boot.startup = function(options) {
 			contentTypeInfo: {} // Map type to {encoding:,extension:}
 		}
 	});
-	if($tw.node) {
+	if(!options.readBrowserTiddlers) {
 		// For writable tiddler files, a hashmap of title to {filepath:,type:,hasMetaFile:}
 		$tw.boot.files = {};
 		// System paths and filenames