set 'archive_fmt' values

These are going to be used to create an unique id for each image.

gallery_dl/extractor/nijie.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2015-2017 Mike Fährmann
+# Copyright 2015-2018 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
@@ -18,6 +18,7 @@ class NijieExtractor(AsynchronousExtractor):
     category = "nijie"
     directory_fmt = ["{category}", "{artist_id}"]
     filename_fmt = "{category}_{artist_id}_{image_id}_p{index:>02}.{extension}"
+    archive_fmt = "{image_id}_{index}"
     cookiedomain = "nijie.info"
     popup_url = "https://nijie.info/view_popup.php?id="