lib: regexp matching in 'subject' and 'from'

the idea is that you can run

% notmuch search subject:/<your-favourite-regexp>/
% notmuch search from:/<your-favourite-regexp>/

or

% notmuch search subject:"your usual phrase search"
% notmuch search from:"usual phrase search"

This feature is only available with recent Xapian, specifically
support for field processors is needed.

It should work with bindings, since it extends the query parser.

This is easy to extend for other value slots, but currently the only
value slots are date, message_id, from, subject, and last_mod. Date is
already searchable;  message_id is left for a followup commit.

This was originally written by Austin Clements, and ported to Xapian
field processors (from Austin's custom query parser) by yours truly.