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()'.
- move HTML request for post pages into its own function
- move gelbooru_v02.py notes extraction to gelbooru.py
since it only works there
- clean up some code
