added support for reading static files from package

cf. discussion at
https://github.com/FND/tiddlywebplugins.bfw/pull/19#issuecomment-21953283

overloading existing configuration parameter for backwards compatibility

I opted not to allow for any kind of complex configuration or cascade of
different source directories and URIs since there's no tangible use case
for that yet (i.e. YAGNI)