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/mastodon.py

@@ -23,8 +23,10 @@ class MastodonExtractor(BaseExtractor):
 
     def __init__(self, match):
         BaseExtractor.__init__(self, match)
-        self.instance = self.root.partition("://")[2]
         self.item = match.group(match.lastindex)
+
+    def _init(self):
+        self.instance = self.root.partition("://")[2]
         self.reblogs = self.config("reblogs", False)
         self.replies = self.config("replies", True)