Delivery to a system command

[foxcpp]

See #157.

For compatibility with existing software, it is a good idea to make our implementation reasonably similar to the Postfix pipe transport: http://www.postfix.org/pipe.8.html

[cuu508]

A workaround that seems to work today is to use a system command filter, and deliver to dummy afterwards. As an example, the following copies the message to /tmp/:

msgpipeline copy_to_tmp {
    default_destination {
        check {
            command /usr/bin/cp /dev/stdin /tmp/{msg_id}.txt
        }

        deliver_to dummy
    }
}