Add group option for arguments

[eugenesvk]

In the similarly named Python module there is a group feature for arguments similar to the command groups introduced in #14.
The Python module also has a great extra feature mutually exclusive group, which by the way also allows to add both commands and arguments to one group.
Would you be willing to add these features (groups; mutually exclusive groups; allow combining arguments/commands in a group)to your module?
Thank you