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/redgifs.py

@@ -24,6 +24,8 @@ class RedgifsExtractor(Extractor):
     def __init__(self, match):
         Extractor.__init__(self, match)
         self.key = match.group(1)
+
+    def _init(self):
         self.api = RedgifsAPI(self)
 
         formats = self.config("format")