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/urlshortener.py

@@ -54,6 +54,7 @@ class UrlshortenerLinkExtractor(UrlshortenerExtractor):
         UrlshortenerExtractor.__init__(self, match)
         self.id = match.group(match.lastindex)
 
+    def _init(self):
         try:
             self.headers = INSTANCES[self.category]["headers"]
         except Exception: