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/__init__.py

@@ -9,7 +9,6 @@
 """Post-processing modules"""
 
 import importlib
-import logging
 
 modules = [
     "classify",
@@ -21,8 +20,6 @@ modules = [
     "zip",
 ]
 
-log = logging.getLogger("postprocessor")
-
 
 def find(name):
     """Return a postprocessor class with the given name"""