adopt printmimestructure from notmuch

notmuch has held the printmimestructure utility in its source repo for a half-dozen years, but we've never shipped it and made it accessible to non-developers.

It's pretty handy for debugging e-mail formatting issues and writing specifications about e-mail, so i want to contribute it to mailscripts.

I've imported the original history from the notmuch repo, cleaned up the source a little bit, renamed it email-print-mime-structure (to match the "conventions" we're starting to establish in mailscripts), and added a manpage for it.

Please adopt! If you don't want me to maintain it here, i'll continue keeping it in the notmuch tree, but i'd really rather it be easier for others to get at.