notmuch search: Add support for a --reverse option to reverse sort order.

Note that the difference between thread results in date order and
thread results in reverse-date order is not simply a matter of
reversing the final results. When sorting in date order, the threads
are sorted by the oldest message in the thread. When sorting in
reverse-date order, the threads are sorted by the newest message in
the thread.

This difference means that we might want an explicit option in the
interface to reverse the order, (even though the default will be to
display the inbox in date order and global searches in reverse-date
order).