Add support for encrypted private keys

[n0nvme]

I got an error when tried to use encrypted private ssh key as identity

Expected Behavior

ask for passphrase then use it as usual

Actual Behavior

Failed to parse key: ssh: cannot decode encrypted private keys

Steps to reproduce behavior

ssh-chat --verbose --bind ":22" --identity ~/.ssh/id_rsa

where ~/.ssh/id_rsa ecnrypted(generated with passphrase) private key

Additional Comments

[shazow]

We had some code that used to work with this, but looks like it's no longer working:
https://github.com/shazow/ssh-chat/blob/master/cmd/ssh-chat/key.go#L32

[shazow]

Looks like newer ssh keys are no longer x509 encrypted the same way?