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/classify.py

@@ -33,7 +33,7 @@ class ClassifyPP(PostProcessor):
         }
 
     def prepare(self, pathfmt):
-        ext = pathfmt.keywords.get("extension")
+        ext = pathfmt.extension
 
         if ext in self.mapping:
             self._dir = pathfmt.realdirectory + os.sep + self.mapping[ext]