$forward_references includes References: header on forwards

$forward_references includes References: header on forwards

By default a forwarded message does not reference the messages it
contains. This commit introduces a new boolean option
`$forward_references`. When it is set, a forwarded message includes the
`In-Reply-To:` and `References:` headers, just like a reply would. Hence
the forwarded message becomes part of the original thread instead of
starting a new one.

Closes: #252