Integrate directly with Keybase

I'd like to see this integrated directly with [keybase](http://keybase.io/). Right now, I have to:

1. Follow someone on keybase
2. Import their key into gpg
3. Sign their key in gpg
4. `gitcrypt add-gpg-user ...`

I'd like to propose additional functionality that would permit:

1. Follow someone on keybase
2. `gitcrypt add-keybase-user ...`