improve output of active post processor modules

gallery_dl/postprocessor/common.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2018 Mike Fährmann
+# Copyright 2018-2019 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
@@ -23,3 +23,6 @@ class PostProcessor():
 
     def finalize(self):
         """Cleanup"""
+
+    def __repr__(self):
+        return self.__class__.__name__