set 'archive_fmt' values

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

gallery_dl/extractor/imgth.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 ImgthGalleryExtractor(Extractor):
     subcategory = "gallery"
     directory_fmt = ["{category}", "{gallery_id} {title}"]
     filename_fmt = "{category}_{gallery_id}_{num:>03}.{extension}"
+    archive_fmt = "{gallery_id}_{num}"
     pattern = [r"(?:https?://)?imgth\.com/gallery/(\d+)"]
     test = [("http://imgth.com/gallery/37/wallpaper-anime", {
         "url": "4ae1d281ca2b48952cf5cca57e9914402ad72748",