Use email instead of username during setup

During `git hub setup`, it would be nice to provide authentication via email + password to be congruent with signing in to GitHub (and avoid possible confusion).

If this is not possible, maybe make it clear when asking for the username?
i.e. instead of
```
GitHub username [gautam]:
```
maybe we could show
```
GitHub username (not email):
```
Also state in the manpage that the username and not email is expected.