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/inkbunny.py

@@ -24,8 +24,7 @@ class InkbunnyExtractor(Extractor):
     archive_fmt = "{file_id}"
     root = "https://inkbunny.net"
 
-    def __init__(self, match):
-        Extractor.__init__(self, match)
+    def _init(self):
         self.api = InkbunnyAPI(self)
 
     def items(self):