support other key hosts beyond github

right now we use github because: https://github.com/maxogden.keys

but we should be able to use any scheme where you can type a username and it can figure out how to get a public key. it should probably prompt the user and double check that they wanna trust the public key from the domain it resolves, similar to what keybase does.

keybase might be a good next one to support actually, as they have a CORS enabled endpoint to get the pgp key for any user