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/imagefap.py

@@ -23,9 +23,8 @@ class ImagefapExtractor(Extractor):
     archive_fmt = "{gallery_id}_{image_id}"
     request_interval = (2.0, 4.0)
 
-    def __init__(self, match):
-        Extractor.__init__(self, match)
-        self.session.headers["Referer"] = self.root
+    def _init(self):
+        self.session.headers["Referer"] = self.root + "/"
 
     def request(self, url, **kwargs):
         response = Extractor.request(self, url, **kwargs)