Add port to smtp config

Currently, it is not possible to configure the smtp port to use for sending email using boss_mail. Furthermore, it can be practical to be able to configure tls specifically, in some cases not using the currently hard-coded if_available setting. This patch makes available two new config options, mail_relay_port and mail_relay_use_tls, the former which takes an integer and the latter which takes either of the atoms if_available, always and never.