Support for repo update command

[ravensorb]

The problem I'm trying to solve: I have a series of repos that I would like to switch from private to public once I get to a certain point in my development. I know its possible to create a new repo with public/private -- what would be great is for there to be a command that allows me to easily update an existing repo

How I imagine hub could expose this functionality: gh repo update --public <name>/<repo> or gh repo update --description "Some updated description" <name>/<repo>

[BLasan]

@ravensorb Can I work on this?