Instead of a strict list of (URL, RESULTS)-tuples, extractor result
tests can now be a single (URL, RESULTS)-tuple, if it's just one test,
and "only matching" tests can now be a simple string.
The same filter infrastructure that can be applied to image URLS now
also works for manga chapters and other delegated URLs.
TODO: actually provide any metadata (currently supported is only
deviantart and imagefap).