Allow customisation of headers stripped by mutt_prepare_template()

[benh]

I mail out a git patch series by sending it to a mailbox, moving the messages into my postponed folder, and then recovering them to edit and send.

git-format-patch generates a thread by setting Message-ID on the first message and References & In-Reply-To on the subsequent message. Unfortunately, mutt strips Message-ID when retrieving postponed messages, so the thread ends up broken.

Please change mutt_prepare_template() so that the set of stripped headers can be customised.