[nijie] fix dojin extraction

- correctly extract artist_id
- set extension to "jpg" if it was empty and let filetype checks do
  the rest

test/test_extractors.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python3
 # -*- 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
@@ -20,6 +20,7 @@ SKIP = {
     # temporary issues
     "chronos",
     "coreimg",
+    "hosturimage",
     "yeet",
 }