Add a script to list all SSH keys on an appliance

For GitHub Enterprise.

@gjtorikian @pengwynn Could I get your :eyes: on this?
:+1: for ENV vars. I'd also suggest using an access token instead of user/pass. [This example][example] might help.

[example]: https://github.com/pengwynn/dotfiles/blob/master/bin/hissuegram#L18-L31