emacs: Allow functions in notmuch-search-result-format

emacs: Allow functions in notmuch-search-result-format

If the car of an element in notmuch-search-result-format is a
function, insert the result of calling the function into the buffer.

This allows a user to generate custom fields in the output of a search
result. For example, with:

(defun -notmuch-result-flags (format-string result)
  (let ((tags-to-letters '(("flagged" . "!")
			   ("unread" . "u")
			   ("mine" . "m")
			   ("sent" . "s")
			   ("replied" . "r")))
	(tags (plist-get result :tags)))

    (format format-string
	    (mapconcat (lambda (t2l)
			 (if (member (car t2l) tags)
			     (cdr t2l)
			   " "))
		       tags-to-letters ""))))

(setq notmuch-search-result-format '((-notmuch-result-flags . "%s ")
				     ("date" . "%12s ")
				     ("count" . "%9s ")
				     ("authors" . "%-30s ")
				     ("subject" . "%s ")
				     ("tags" . "(%s)")))

The first few characters on each line of the search result are used to
show information about some significant tags associated with the
thread.