Add format variables for the default formats for the various modes

(tree_mode_format and so on) and add a -a flag to display-message to
list variables with values.

window-buffer.c

@@ -41,6 +41,7 @@ static void		 window_buffer_key(struct window_mode_entry *,
 
 const struct window_mode window_buffer_mode = {
 	.name = "buffer-mode",
+	.default_format = WINDOW_BUFFER_DEFAULT_FORMAT,
 
 	.init = window_buffer_init,
 	.free = window_buffer_free,