Retrieve OAuth token with minimal pain

This should ease the pain of having to go through the web flow to retrieve OAuth token as @jbarnette mentions in https://github.com/boxen/boxen/pull/101#issuecomment-20959887.

![ebnxxwg](https://f.cloud.github.com/assets/550256/798430/4db2a8de-ed60-11e2-93b0-f9d0d9137af8.gif)

Keep flowin'.