let extractors opt-out of cookie option usage

useful to avoid sending unnecessary cookies when all authentication
is done through OAuth tokens

gallery_dl/extractor/smugmug.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2018-2019 Mike Fährmann
+# Copyright 2018-2020 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
@@ -21,6 +21,7 @@ class SmugmugExtractor(Extractor):
     category = "smugmug"
     filename_fmt = ("{category}_{User[NickName]:?/_/}"
                     "{Image[UploadKey]}_{Image[ImageKey]}.{extension}")
+    cookiedomain = None
     empty_user = {
         "Uri": "",
         "ResponseLevel": "Public",