New part output handling as option to notmuch-show.

Outputting of single MIME parts is moved to an option of notmuch show,
instead of being handled in it's own sub-command.  The recent rework
of multipart mime allowed for this change but consolidating part
handling into a single recursive function (show_message_part) that
includes formatting.  This allows for far simpler handling single
output of a single part, including formatting.