set 'archive_fmt' values

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

gallery_dl/extractor/senmanga.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2016-2017 Mike Fährmann
+# Copyright 2016-2018 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 SenmangaChapterExtractor(Extractor):
     subcategory = "chapter"
     directory_fmt = ["{category}", "{manga}", "{chapter_string}"]
     filename_fmt = "{manga}_{chapter_string}_{page:>03}.{extension}"
+    archive_fmt = "{manga}_{chapter_string}_{page}"
     pattern = [r"(?:https?://)?raw\.senmanga\.com/([^/]+/[^/]+)"]
     test = [
         ("http://raw.senmanga.com/Bokura-wa-Minna-Kawaisou/37A/1", {