Add 'notmuch count' command to show the count of matching messages

Getting the count of matching threads or messages is a fairly
expensive operation. Xapian provides a very efficient mechanism that
returns an approximate value, so use that for this new command.

This returns the number of matching messages, not threads, as that is
cheap to compute.

Signed-off-by: Keith Packard <keithp@keithp.com>