Pinterest access tokens are rate limited at 200 requests per
hour (or maybe per 2 or 3 hours?) so having just one access token
for all users isn't going to work in the long run.
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)