support 'filter' option for post processors (#1460)

gallery_dl/postprocessor/mtime.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2019-2020 Mike Fährmann
+# Copyright 2019-2021 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
@@ -17,7 +17,7 @@ class MtimePP(PostProcessor):
     def __init__(self, job, options):
         PostProcessor.__init__(self, job)
         self.key = options.get("key", "date")
-        job.hooks["file"].append(self.run)
+        job.register_hooks({"file": self.run}, options)
 
     def run(self, pathfmt):
         mtime = pathfmt.kwdict.get(self.key)