Add more options to configure the smtplib implementation for sending
notification emails.

The port can be changed using the new smtp-port option.

Encryption can be configured using smtp-use-ssl and smtp-use-starttls.
Keep in mind that you usually also need to change the port when enabling
either of these options.

Authentication can be configured using smtp-user and smtp-password.
Authentication is disabled if either of these values is empty.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>