v2.0.0

• Now uses R6 classes instead of proto classes (#25).
  The typical user shouldn't need to change any argparse code
  but I did make private the previously public python_code
  element of the object returned by ArgumentParser().
• Parsers now support add_subparsers (#14).
  Suggestion of Zebulun Arendsee.
• Parsers now support add_argument_group (#26).
  Suggestion of Dario Beraldi.
• Parsers now support add_mutually_exclusive_group (#23).
  Suggestion of Vince Reuter.