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)

test/test_results.py

@@ -218,8 +218,8 @@ class TestPathfmt():
         self.hashobj = hashobj
         self.path = ""
         self.size = 0
-        self.keywords = {}
-        self.has_extension = True
+        self.kwdict = {}
+        self.extension = "jpg"
 
     def __enter__(self):
         return self