support for multipart/alternative and multipart/mixed

support for multipart/alternative and multipart/mixed

This work is based on commit 5b4f7a1342 from @somini and was discussed
at <https://github.com/neomutt/neomutt/issues/587>. As of now, one can
tag entries in the compose menu and use <group-alternatives> to group
those entries into one. If all the entries are tagged (including the
fundamental part), then the email become a multipart/alternative one,
otherwise it becomes a multipart/mixed one.

There is some weird things to be noted, though: when postponing a
multipart/alternative mail and then recall it, those entries get
split up. This does not occurred for multipart/mixed mails.