emacs: Show all multipart/alternative parts by default.

emacs: Show all multipart/alternative parts by default.

This is patch is a temporary work-around for a slight regression that
popped up in the part handling reorganization.  Currently, text/plain
parts are always preferred, if present, over other non-text/plain
parts in multipart/alternative.  However, this means that if there is
a blank text/plain part, no content will be displayed.

One way to get around this is to set the
"notmuch-show-all-multipart/alternative-parts" customization variable
to True ('t'), which will cause all parts to always be displayed.

Since we want to move forward with the next release, we're going to
set this variable true by default, to make sure that no content is
unretrievably hidden from the user.  Once we come up with a better
solution for easy display of hidden parts we can set this back to a
default value of 'nil'.