feature: support gpg keys (with subkeys!)

Thanks for making the command line glorious.

It'd really rock if melt would support the correct-horse-battery-staple-ification of GPG keys in addition to the subset of SSH keys it supports currently.

What a win for good security hygiene on the internet, if we could have GPG keys with SSH subkeys all safely protected with a classic Charm ui/ux win for ease of use and simplification.... :-)

Please close if out of scope.