Fix #33 - cmd2 choices_method

[eljeffeg]

Update Katana for changes in cmd2 >= 2.0. See https://github.com/python-cmd2/cmd2/releases/tag/2.0.0 (June 2021)

• Fixes: TypeError: __init__() got an unexpected keyword argument 'choices_method' bug
• Note: Existing users that update Katana will need to update cmd2 or they'll get a similar error but 'choices_provider'

Note to reviewer. I haven't tested this beyond making sure the app runs katana -h, where it would fail prior.