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).
This commit mostly replaces all minus-signs ('-') in keyword names with
underscores ('_') to allow them to be used in filter-expressions. For
example 'gallery-id' got renamed to 'gallery_id'.
(It is theoretically possible to access any variable, regardless of its
name, with 'locals()["NAME"]', but that seems a bit too convoluted if
just 'NAME' could be enough)
Mike Fährmann