Add subcommand support

[chrisdone-artificial]

hell examples/41-commands.hell --help
Usage: <interactive> COMMAND

Available options:
  -h,--help                Show this help text

Available commands:
  add                      Add the thing
  remove                   
  list                      

hell examples/41-commands.hell add --help
Usage: <interactive> add FILE

Available options:
  FILE                     File to add
  -h,--help                Show this help text

hell examples/41-commands.hell add foo
Adding foo