Add support for 'UseKeychain'

[ocean90]

macOS Sierra 10.12.2 includes an new option called UseKeychain:

$ man ssh_config
     UseKeychain
             On macOS, specifies whether the system should search for passphrases in the user's keychain when attempting to use a par-
             ticular key. When the passphrase is provided by the user, this option also specifies whether the passphrase should be
             stored into the keychain once it has been verified to be correct.  The argument must be ``yes'' or ``no''.  The default is
             ``no''.

Disclaimer: I don't know Go and don't know how to run the tests. :) Tests are passing 💪🏼

[moul]

Hi @ocean90, thank you for this PR, and congratulations, everything seems good to me ;)