RFC/PATCH emacs attachment handling

[Matthieu Lemerre]

> Finally, I have also mapped the key "o" (other/open with) on a button to
> open with user chosen program. This could be split out into a separate
> patch if preferred.

This is great! In particular many people send PDF attachments to me with
mime-type attachment/octet-stream, and notmuch could not view them and
only offered to save them... I have wanted this functionality for long!

Matthieu