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/manganelo.py

@@ -21,7 +21,9 @@ class ManganeloBase():
     def __init__(self, match):
         domain, path = match.groups()
         super().__init__(match, "https://" + domain + path)
-        self.session.headers['Referer'] = self.root
+
+    def _init(self):
+        self.session.headers['Referer'] = self.root + "/"
 
         if self._match_chapter is None:
             ManganeloBase._match_chapter = re.compile(