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/gfycat.py

@@ -24,6 +24,7 @@ class GfycatExtractor(Extractor):
         Extractor.__init__(self, match)
         self.key = match.group(1).lower()
 
+    def _init(self):
         formats = self.config("format")
         if formats is None:
             formats = ("mp4", "webm", "mobile", "gif")