`hk authorize`: two factor support for paranoid/privileged actions

As originally described by the design in #171, implements a command called `hk authorize` that uses heroku-agent to procure and store a token that will bypass two factor checks on paranoid apps and other privileged actions for a short period of time.

Here's a sample interaction (slightly sanitized):

``` bash
$ hk env -a paranoid
error: A second authentication factor or pre-authorization is required for this request. Your account has either two-factor or a Yubikey registered. Authorize with `hk authorize`.

$ hk authorize
Enter two-factor auth code: ccccccdlnndfutjnddejbcceriigfjjbvulurgntjvum
Authorization successful.

$ hk env -a paranoid
RACK_ENV=production
```

Fixes #171.