emacs: Support for customizing search result display

This patch helps in customizing search result display similar to
mutt's index_format. The customization is done by defining an alist as
below:

(setq notmuch-search-result-format '(("date" . "%s ")
				     ("authors" . "%-40s ")
				     ("subject" . "%s ")))

The supported keywords are date, count, authors, subject and tags.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@gmail.com>
Signed-off-by: David Edmondson <dme@dme.org>