emacs: Allow functions in notmuch-{tree,unthreaded}-result-format

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

emacs/notmuch-tree.el

@@ -885,6 +885,9 @@ unchanged ADDRESS if parsing fails."
      ((listp field)
       (format format-string (notmuch-tree-format-field-list field msg)))
 
+     ((functionp field)
+      (funcall field format-string msg))
+
      ((string-equal field "date")
       (let ((face (if match
 		      'notmuch-tree-match-date-face