allow sending mail via external programs

[aspiers]

Ideally it would be possible to send mail via external programs, like msmtp. In fact, I wonder if it would make more sense for pius to rely on external programs for sending mail? That way it wouldn't have to duplicate that functionality in libpius.mailer.

[jaymzh]

#29 provides an option that should work for you. However, most of what libpius.mailer does is actually assemble the messages correctly, which we'd always want to do.