Migrate to Cobra's built-in MutuallyExclusive flag helper

[mislav]

This looks more declarative but the error message is abysmal:

%  gh repo create --public --private
if any flags in the group [public private internal] are set none of the others can be; [private public] were all set

TODO:

• figure out if we can transform Cobra's error message into something like before
• fix tests

[rsteube]

Think the only option here at the moment would be to set SilenceErrors and do some matching and regex transformation on the err returned by Execute for that (yuck).

[mislav]

Closing this; switching to Cobra was a downgrade and didn't improve anything other than brevity of code.