more robust PAGINATION_(URL|SAVE_AS) support

- add base_name and number_seperator to context to give more flexibility
  when naming things

pelican/writers.py

@@ -150,7 +150,7 @@ class Writer(object):
         # check paginated
         paginated = paginated or {}
         if paginated:
-            name_root, ext = os.path.splitext(name)
+            name_root = os.path.splitext(name)[0]
 
             # pagination needed, init paginators
             paginators = {}