[improvement] support openssh format privkey

[kmahyyg]

since ed25519 is much more recommended now, can this app support ed25519 algo. private key?
the default key of ed25519 is saved in openssh format instead of pem.

[ellbur]

As an alternative to adding support for OpenSSH format, the wiki page Key-based-ssh-Authentication could be updated to show how OpenSSH keys can be converted to PEM:

ssh-keygen -p -f ~/.ssh/some_key_file -m PEM

(mosh-chrome currently does not warn/error if it cannot read the key format.)