Write a "notmuch tag" command to add/remove tags from messages
matching a search query.

Write a "notmuch show" that displays a single thread.

Fix to use the *last* Message-ID header if multiple such headers are
encountered, (I noticed this is one thing that kept me from seeing the
same message-ID values as sup).

Think about this race condition:

	A client executes "notmuch search"
	Then executes "notmuch show" on a thread
	While user is reading, new mail is added to database for the thread
	Client asks for the thread to be archived.

   The bug here is that email that was never read will be
   archived. That's bad. With the command set above, the user can
   avoid the problem by just not running "notmuch new" while reading
   mail, but the same problems exists with the API. One possible
   solution would be to store an additional timestamp with each mail
   document for the time it was added to the database. Then searches
   could return a timestamp, and the client could pass that same
   timestamp back to the archive command to not modify any messages
   with a timestamp newer than what's passed.