[RFC] Added unattended install of postfix

Fixes #37, verified it can send messages and implements sendmail command.

We could potentially add extra functionality in the following ways:

0. A template for `/etc/aliases` and a var `mail.postmaster`, to define an off-server email address to which any system emails should be sent (rather than going unread and filling up the server spool)
0. A template for `/etc/postfix/main.cf` and smtp credentials under a `mail` dict to relay all email to an external smtp server (eg: gmail); or alternately install something like nullmailer (instead of postfix) that *only* relays mail

The first item would be easy and ensure the server operator gets system notices. I honestly don't know if the second would be too useful for most cases.