implement 'mtime' post-processor (#332)

This can set a file's modification time according to a UNIX timestamp
or a datetime object from its metadata.

gallery_dl/postprocessor/__init__.py

@@ -15,6 +15,7 @@ modules = [
     "classify",
     "exec",
     "metadata",
+    "mtime",
     "ugoira",
     "zip",
 ]

gallery_dl/postprocessor/mtime.py

@@ -0,0 +1,27 @@
+# -*- coding: utf-8 -*-
+
+# Copyright 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
+# published by the Free Software Foundation.
+
+"""Use metadata as file modification time"""
+
+from .common import PostProcessor
+from ..text import parse_int
+
+
+class MtimePP(PostProcessor):
+
+    def __init__(self, pathfmt, options):
+        PostProcessor.__init__(self)
+        self.key = options.get("key", "date")
+
+    def run(self, pathfmt):
+        mtime = pathfmt.keywords.get(self.key)
+        ts = getattr(mtime, "timestamp", None)
+        pathfmt.keywords["_mtime"] = ts() if ts else parse_int(mtime)
+
+
+__postprocessor__ = MtimePP