emacs: show: let the user override the mime-type of an attachment

This allows the user to override the mime-type of a part in the show
buffer. This takes the simple option of displaying the part with the
specified mime-type in its own buffer (in view mode). This avoids
further complicating the part handling code.

Bound to ". m" (i.e., m in the part map). Then the user can either
enter a mime-type (with completion to all mime types that mailcap (and
thus notmuch) knows about, or press return for the default choice of
text/plain.