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/vipergirls.py

@@ -26,9 +26,8 @@ class VipergirlsExtractor(Extractor):
     cookies_domain = ".vipergirls.to"
     cookies_names = ("vg_userid", "vg_password")
 
-    def __init__(self, match):
-        Extractor.__init__(self, match)
-        self.session.headers["Referer"] = self.root
+    def _init(self):
+        self.session.headers["Referer"] = self.root + "/"
 
     def items(self):
         self.login()