added flag to Cntlr and FileSource which disables non-local fetching

@hefischer, while working on some GAE tasks I was looking for a way to disable all http requests and force Arelle to look only into the cache directory.  Not seeing how to do this I added a flag to the ```Cntlr``` class and the associated code.  I'm not at all married to this code, but wanted to show you so we could discuss how to enable "local only" behavior.  The result of this code is if the file is not cached it will fail and give a message with the file that's missed.