Add support for password-protected ssh key

[praetp]

My ssh key is password protected (which is fundamental security practice).
Alas, sshping does not support this yet AFAICS.

[tsathoqqua]

Totally agree. It's fundamental that this supports passphrases for identity files. This is not useful unless it does. I personally have 0 keys without passphrases.

[wwalker]

Many of us have our keys protected, with passphrases; but, we run a local ssh-agent, so never have to do any special authentication steps with sshping.

However, I totally agree, it should be supported, however also, since I think that sshping makes multiple connections during a run, waiting for a person to enter authentication data each time may very much confuse the reported timings (as sshping would not know that the delay was for authentication).