auth

[jonschlinkert]

@doowb I know you've done a lot of stuff with the github API recently, I just pushed this up based on the conversation in update/update#10, but there should probably be an auth task that runs before the default task. Maybe even a prompt-auth task as well.

cc @jamen

[doowb]

Should be able to use something like ask-for-github-auth... probably needs a refactor to bring it up-to-date, but the idea is to allow users to use their username/password combo or a github authentication token.