* [pornpics] add category and listing extractors
Add support for:
- Category pages like /ass/, /milf/, /blonde/ etc.
- Listing pages like /popular/, /recent/, /rating/, /likes/, /views/, /comments/
Category pages use JSON pagination like tags/search.
Listing pages don't support JSON pagination and use different HTML structure.
* [pornpics] simplify category pattern via class ordering
- Move PornpicsCategoryExtractor after PornpicsListingExtractor
so it acts as catch-all, eliminating need for negative lookahead
- Use list comprehension in PornpicsListingExtractor.galleries()
* update docs/supportedsites
Introduce an 'initialize()' function that does the actual init
(session, cookies, config options) and can called separately from
the constructor __init__().
This allows, for example, to adjust config access inside a Job
before most of it already happened when calling 'extractor.find()'.
