Add the magic to allow searches such as "tag:inbox".

The key for this is call add_boolean_prefix on the QueryParser
object. That tells the query parser to take something like "tag:inbox"
and transform it into the "Linbox" term and do what it needs to do to
make this term a requirement of the search. We're starting to have a
real system here.

Also, I didn't want to expose the ugly name of "msgid" to the user, so
we add a prefix name of simply "id" instead.