set 'archive_fmt' values

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

gallery_dl/extractor/2chan.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2017 Mike Fährmann
+# Copyright 2017-2018 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
@@ -17,8 +17,10 @@ class FutabaThreadExtractor(Extractor):
     category = "2chan"
     subcategory = "thread"
     directory_fmt = ["{category}", "{board_name}", "{thread}"]
-    pattern = [r"(?:https?://)?(([^.]+)\.2chan\.net/([^/]+)/res/(\d+))"]
+    filename_fmt = "{tim}.{extension}"
+    archive_fmt = "{board}_{thread}_{tim}"
     urlfmt = "https://{server}.2chan.net/{board}/src/{filename}"
+    pattern = [r"(?:https?://)?(([^.]+)\.2chan\.net/([^/]+)/res/(\d+))"]
     test = [("http://dec.2chan.net/70/res/947.htm", {
         "url": "c5c12b80b290e224b6758507b3bb952044f4595b",
         "keyword": "4bd22e7a9c3636faecd6ea7082509e8655e10dd0",