make 'path' and 'keywords' available in logging messages

Wrap all loggers used by job, extractor, downloader, and postprocessor
objects into a (custom) LoggerAdapter that provides access to the
underlying job, extractor, pathfmt, and kwdict objects and their
properties.

__init__() signatures for all downloader and postprocessor classes have
been changed to take the current Job object as their first argument,
instead of the current extractor or pathfmt.

(#574, #575)

gallery_dl/postprocessor/exec.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2018-2019 Mike Fährmann
+# Copyright 2018-2020 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
@@ -23,8 +23,8 @@ else:
 
 class ExecPP(PostProcessor):
 
-    def __init__(self, pathfmt, options):
-        PostProcessor.__init__(self)
+    def __init__(self, job, options):
+        PostProcessor.__init__(self, job)
         args = options["command"]
         final = options.get("final", False)