Update volume extension to use the new interface

New interface from #85

src/rocker/volume_extension.py

@@ -61,7 +61,7 @@ def get_docker_args(self, cli_args):
         return ' '.join(args)
 
     @staticmethod
-    def register_arguments(parser):
+    def register_arguments(parser, defaults={}):
         parser.add_argument(Volume.ARG_ROCKER_VOLUME,
             metavar='HOST-DIR[:CONTAINER-DIR[:OPTIONS]]',
             type=str,