update PathFormat class

- change 'has_extension' from a simple flag/bool to a field that
  contains the original filename extension
- rename 'keywords' to 'kwdict' and some other stuff as well
- inline 'adjust_path()'
- put enumeration index before filename extension (#306)

gallery_dl/postprocessor/mtime.py

@@ -19,9 +19,9 @@ class MtimePP(PostProcessor):
         self.key = options.get("key", "date")
 
     def run(self, pathfmt):
-        mtime = pathfmt.keywords.get(self.key)
+        mtime = pathfmt.kwdict.get(self.key)
         ts = getattr(mtime, "timestamp", None)
-        pathfmt.keywords["_mtime"] = ts() if ts else parse_int(mtime)
+        pathfmt.kwdict["_mtime"] = ts() if ts else parse_int(mtime)
 
 
 __postprocessor__ = MtimePP