Allow setting bundle to use in file utils

Our cocos app has an apparently uncommon use case where we need to be able to download non-essential assets for our app.  The best solution (for ios) I've been able to come up with is to download a bundle and load it for the cocos environment, but it doesn't currently support using any bundle other than the main one from what I've seen. 

This PR adds the ability to do just that, allowing the client code to call something like this to set it:
```
((cocos2d::FileUtilsApple*)(cocos2d::FileUtils::getInstance()))->setBundle(bundle);
```
As it is something specific to apple, I added the change to the FileUtilsApple, which forces you to cast the FileUtils to the subclass for apple. I'm not a big fan of this, so I'd welcome ideas to avoid having to do it.

This is an uncommon use case, but I've seen other people trying to do the same thing online, so I thought it'd be interesting to get it on the main repository. Let me know if there is any changes/improvements needed to get this  merged.