Add a Sender and Reply-To header to the Emailer transformer

It would be useful to extend the Emailer to include extra originator fields as per https://tools.ietf.org/html/rfc5322#section-3.6.2

At the moment it has From, To, CC and BCC fields. Sender and Reply-To fields would allow the email to be sent on behalf of someone and replies to be directed elsewhere.

from            =   "From:" mailbox-list      sender          =   "Sender:" mailbox  reply-to        =   "Reply-To:" address-list