add support for auth_token

Simple addition for using an access token instead of username/password to authenticate. Access tokens can be generated by any user on https://github.com/settings/applications.

Access tokens are much preferred over username/password because they prevent the user from typing/hardcoding their password anywhere (which could end up in plain-text logs, etc). And if a token gets leaked, it can easily be revoked, and the attacker cannot use the token to change the Github password and lock-out the actual user.