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/pornpics.py

@@ -23,7 +23,9 @@ class PornpicsExtractor(Extractor):
     def __init__(self, match):
         super().__init__(match)
         self.item = match.group(1)
-        self.session.headers["Referer"] = self.root
+
+    def _init(self):
+        self.session.headers["Referer"] = self.root + "/"
 
     def items(self):
         for gallery in self.galleries():