Feature request: Support TOTP SSH 2FA authentication

[tjcampanella]

Recently, I have been adding a little extra bit of security for some of my servers. I added 2FA to ssh logins on my Debian boxes using libpam-google-authenticator. It would be nice if this action supported this use case. I don't think it would require that big of a change. An possible solution would be adding a new optional field to accept the secret key and then using a command line tool generate a code from the secret key when logging into the server. Not sure how much this feature is in demand but figured I would put it out there. Thanks.

[appleboy]

ref PR: golang/crypto#89