gkeys.cli: Convert to Argparse sub-parsers

This lets us use the method docstrings for more detailed help:

  $ gkeys --help
  ...
  positional arguments:
    {listseed,addseed,removeseed,moveseed,...}
                          actions
      listseed            Pretty-print the selected seed file(s)
      addseed             Add a key to the selected seed file(s)
      removeseed          Remove a key from the selected seed file(s)
      moveseed            Move keys between seed files
  ...

  $ gkeys listseed --help
  usage: gkeys listseed [-h]

  Pretty-print the selected seed file(s)

  optional arguments:
    -h, --help  show this help message and exit