Support sub commands without arguments

[David Santiago Soto]

Right now we have to give call subc (rosrun sub8_missions move_command) with dummy args for comands that don't take any arguments like stop, level_off, zrp.

Success is being able to run

subc stop

or

subc zrp

without a hideous traceback message.

This task is open to new people!

[David Santiago Soto]

@kingkevlar, you probably just have to catch the exception (out of bounds indexing of the args from argparse) and check to make sure that the command was one of the ones we know to take no arguments.