notmuch search: Add --first and --max-threads options for incremental search.

This time, things are actually tested. The current results aren't
exactly the same as previous results since the incremental search
doesn't necessarily see all the new messages that pertain to the
thread. This means that some author names are missing.

I plan to fix this by doing an additional database search for all
messages in each thread. Of course, this will also be different than
before since now the result will display *all* authors in the thread
(rather than only those that matched the search) but that's probably
what we really want to display anyway.