Turn auto-completion into a first class command

[michaelklishin]

Autocompletion currently uses a magic argument and requires an explicit "separation" of the positional argument:

rabbitmqctl --auto-complete -- sta
# => start_app
# => status

instead, it should be a first class command that can be documented in help:

rabbitmqctl autocomplete sta
# => start_app
# => status

Per discussion with @gerhard