Add sub-commands for `gita group`

[hamiltont]

IMO the command syntax is confusing - you have to pass a list of groups before you get any indication that a name will be requested. I spent 4-5min reading the help text thinking I was missing something, because there was no indication a name would be requested.

At a minimum, update the help text to say "You will be prompted for the group name". However, ideally you woudl support a --name arg before the positional args, so the entire command could be scripted if needed

[nosarthur]

Thanks for the suggestion. Maybe it's better to add subcommand to gita group such as add, rm, etc

[hamiltont]

A sub-subcommand does make sense - gita group ls, gita group add , etc seems quite clear