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, default={}):
  parser.add_argument(Volume.ARG_ROCKER_VOLUME,
  metavar='HOST-DIR[:CONTAINER-DIR[:OPTIONS]]',
  type=str,