notmuch-query.el: new file to support access to the notmuch database.

notmuch-query.el: new file to support access to the notmuch database.

Initially this file provides one main function
notmuch-query-get-threads, which takes a set of search terms, and
returns a parsed set of matching threads as a lisp data structure.

A set of notmuch-query-map-* functions are provided to help map
functions over the data structure.

The function notmuch-query-get-message-ids uses this machinery to get
the set of message-ids matching a query.

Edited-by: Carl Worth <cworth@cworth.org>: Change comment syntax,
(";;" rather than ";" to make emacs-lisp mode happy), and eliminate
some excess whitespace, as suggested by David Edmonson.