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()'.
E.g. `hot4lexi` would start downloading the `hot` section by mistake
This happened because the regex had a negative lookahead for the category names, but didn't ensure that they where followed by either end-of-string or a slash.
