notmuch reply: Use GMime to construct the header for the reply.

notmuch reply: Use GMime to construct the header for the reply.

The advantage here is that we actually get the necessary folding of
long headers, (particularly the References header, but also things
like Subject). This also gives us parsed recipient addresses so that
we can easily elide the sender's address(es) from the recipient list
(just as soon as we have a configured value for the recipient's
address(es)).