Export NetDialTimeout to public

[Elbandi]

Export the netdial function, this allow setup a custom dialer (like tor socks proxy):

dialer, _ := proxy.FromURL("socks5://127.0.0.1:9050", proxy.Direct) 
gomail.NetDialTimeout = func(network, address string, timeout time.Duration) (net.Conn, error) {
  return dialer.Dial(network, address)
}

[ivy]

This should probably be moved out of this var block so no one's confused later by the // Stubbed out for tests. comment.

[pedromorgan]

This is merged in the WIP fork https://github.com/go-mail/gomail/commits/master