Allow AVE files to load other AVE files

The following room should load all the rooms from `minigame.ave`, and send the user to the `start` room of that subgame. If that subgame is won, send the user to `win_minigame`. If gameover is given in the subgame, send the user to `minigameloss`.

```# thisroomid
!! mingame.ave
__WINNER__ => win_minigame
__GAMEOVER__ => minigameloss```