Add SmtpUsername and SmtpPassword fields to conf to allow authenticated SMTP

Authentication will only be attempted if TLS handshake was successful and a username or password was specified.

Fixes #417 
Are we sure this is needed? The net/smtp examples do indeed remove the port from this parameter. Examining the plainAuth code in net/smtp/auth.go, it looks like the host field here is compared to the server's name, which theoretically won't have the port number. So, what happens if you leave the port on? Does authentication fail in that case?