Add --assignee to issue commands

[cryptiklemur]

As a user, it would be nice to be able to pass an --assignee flag to commands like issue:create and issue:take.

So when I want to take an issue, I could say:

$ gush i:take 47 --assignee=aequasi

OR

$ gush i:take 47 -a aequasi

It would also be nice if that could be a global config that could be set in ~/.gush.yml

[cordoval]

closing as this is actually the job of a macro command with bldr, you can use gush i:take and gush i:assign in tandem

[cordoval]

I closed this honoring the desire of @dantleech to keep Gush a micro command set