emacs: Optionally show all parts in multipart/alternative.

emacs: Optionally show all parts in multipart/alternative.

Add a variable `notmuch-show-all-multipart/alternative-parts' that
allows the user to indicate that all candidate sub-parts of a
multipart/alternative part should be shown rather than just the
preferred part. The default is `nil', showing only the preferred part.

This is mostly a debugging aid.

Signed-off-by: Jameson Rollins <jrollins@finestructure.net>