Allow automatic OAuth token creation, like `hub`

When calling a `hub` command that requires GitHub authentication for the first time, the prompt asks for username and password (never saved) to request an OAuth token from GitHub (which is saved) for all future authenticated calls.

I think it's a nice shortcut so the user doesn't have to create a token via the website and copy-paste.

### Example
To resolve the default username and access the user's private repositories: `hub clone -p repo` that resolves to `git clone git@github.com:henrahmagix/repo.git`.
Do we need to specifically check for `message === 'Bad credentials'`? If an error occurs, it should mean authentication has failed for the current authentication details.

Otherwise, I think we should deal with an error that's not related to bad authentication. What other possible messages or codes are there?