`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
```
Flex out the explanatory comment on this no-op
Merge pull request #172 from heroku/brandur-hk-authorize

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