mail.qm - prune MTA-inserted headers from an mbox

mail.qm is designed to make it easy to browse an mbox using a regular text
viewer such as less.

Its main function is to make it easy to look back up from a matched string to
the date and subject email headers so you can find that email in your MUA.

This assumes the text part of an email always comes before HTML and mime64
attachments, but so far that's been the case.
