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/util.py

@@ -726,6 +726,7 @@ class PathFormat():
 
     def set_directory(self, kwdict):
         """Build directory path and create it if necessary"""
+        self.kwdict = kwdict
         windows = os.name == "nt"
 
         # Build path segments by applying 'kwdict' to directory format strings