Support for ed25519 key type

[ismail]

Trying to import ed25519 [0] formatted SSH private/pubkey pair doesn't work, which I think only RSA key-type formatted is supported at the moment? It'd be nice to support ed25519 keys as well.

[0] https://tools.ietf.org/id/draft-ietf-curdle-ssh-ed25519-02.html

[msfjarvis]

Support for modern key ciphers will need a large rewrite that would unfortunately also regress some features so we're not committing to it right now. The issue is also a duplicate of #448 so I'm closing it.