Implement STARTTLS

[ivucica]

This would reduce the impact of PLAIN-only authentication a bit.

Possible approach that I took in my toy server:

cert, err := tls.LoadX509KeyPair("/tmp/goimapd.crt", "/tmp/goimapd.key")
if err != nil {
  fmt.Printf("loadkeys: %s\n", err)
  textconn.PrintfLine("%s BAD Sorry, server-side problem loading certs.", tag, cmd)
  return
}
tlsconfig := &tls.Config{
  Certificates: []tls.Certificate{cert},
}

textconn.PrintfLine("%s OK STARTTLS commencing.", tag)
conn = tls.Server(conn, tlsconfig)
textconn = textproto.NewConn(conn)

It looks like a similar approach could be implemented in imap-server, too: the STARTTLS command could replace c.Rwc with an instance of tls.Server.