Add TLS options for SMTP

[wb7]

Hello,

I am using docker-mailman in combination to https://github.com/mailcow/mailcow-dockerized - and for signing all outgoing mails with DKIM, mailman has to send them authenticated. For security reasons, I do not allow unencrypted authentication on the server (STARTTLS).

I've added some options to enable TLS or SSL when using exim or postfix as MTA. They all default to the standard values according to the mailman docs.

[maxking]

Thanks for the changes, we'll need to fix the CI, but the changes look good to me!