cli: add --include-html option to notmuch show

For my client, the largest bottleneck for displaying large threads is
exporting each html part individually since by default notmuch will not
show the json parts.  For large threads there can be quite a few parts and
each must be exported and decoded one by one.  Also, I then have to deal
with all the crazy charsets which I can do through a library but is a
pain.

Therefore, this patch adds an --include-html option that causes the
text/html parts to be included as part of the output of show.

diff man/man1/notmuch-show.1