Added file marks; fixes issue #94

- Command it_toggle_image_mark (bound to 'm') toggles mark of current
  image
- Command it_navigate_marked (bound to 'N'/'P') can be used to go to
  the next/previous marked image
- When option -o is given, all marked files get printed

commands.h

@@ -54,6 +54,8 @@ bool it_first(arg_t);
 bool it_n_or_last(arg_t);
 bool i_navigate_frame(arg_t);
 bool i_toggle_animation(arg_t);
+bool it_toggle_image_mark(arg_t);
+bool it_navigate_marked(arg_t);
 bool it_scroll_move(arg_t);
 bool it_scroll_screen(arg_t);
 bool i_scroll_to_edge(arg_t);