Add a preliminary "notmuch search" command.

Add a preliminary "notmuch search" command.

This isn't behaving at all like it's documented yet, (for example,
it's returning message IDs not thread IDs[*]). In fact, the output
code is just a copy of the body of "notmuch dump", so all you
get for now is message ID and tags.

But this should at least be enough to start exercising the query
functionality, (which is currently very buggy).

[*] I'll want to convert the databse to store thread documents
before fixing that.