implement '--images' and '--chapters' options

- the former '--items' has been renamed to '--chapters'
- #6

gallery_dl/exception.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2015, 2016 Mike Fährmann
+# Copyright 2015-2017 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
@@ -21,3 +21,7 @@ class AuthorizationError(Exception):
 
 class NotFoundError(Exception):
     """Requested resource (gallery/image) does not exist"""
+
+
+class StopExtraction(Exception):
+    """Extraction should stop"""