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()'.

test/test_downloader.py

@@ -34,6 +34,7 @@ class FakeJob():
 
     def __init__(self):
         self.extractor = extractor.find("test:")
+        self.extractor.initialize()
         self.pathfmt = path.PathFormat(self.extractor)
         self.out = output.NullOutput()
         self.get_logger = logging.getLogger