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/foolfuuka.py

@@ -22,10 +22,12 @@ class FoolfuukaExtractor(BaseExtractor):
 
     def __init__(self, match):
         BaseExtractor.__init__(self, match)
-        self.session.headers["Referer"] = self.root
         if self.category == "b4k":
             self.remote = self._remote_direct
 
+    def _init(self):
+        self.session.headers["Referer"] = self.root + "/"
+
     def items(self):
         yield Message.Directory, self.metadata()
         for post in self.posts():