decouple extractor initialization

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()'.

gallery_dl/extractor/skeb.py

@@ -22,6 +22,8 @@ class SkebExtractor(Extractor):
     def __init__(self, match):
         Extractor.__init__(self, match)
         self.user_name = match.group(1)
+
+    def _init(self):
         self.thumbnails = self.config("thumbnails", False)
         self.article = self.config("article", False)