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/photobucket.py

@@ -48,9 +48,10 @@ class PhotobucketAlbumExtractor(Extractor):
     )
 
     def __init__(self, match):
-        Extractor.__init__(self, match)
-        self.album_path = ""
         self.root = "https://" + match.group(1)
+        Extractor.__init__(self, match)
+
+    def _init(self):
         self.session.headers["Referer"] = self.url
 
     def items(self):
@@ -129,6 +130,8 @@ class PhotobucketImageExtractor(Extractor):
         Extractor.__init__(self, match)
         self.user = match.group(1) or match.group(3)
         self.media_id = match.group(2)
+
+    def _init(self):
         self.session.headers["Referer"] = self.url
 
     def items(self):