Environment Variables in resources.cfg

[NinjaSaru]

I've seen posts from a long time ago when people mentioned being able to use environment variables in resources.cfg (and plugins.cfg). Why not go ahead add this? It would be really handy when trying to run add-on or third-party ogre apps. I've run into several of them that needed lots of file moving or cfg tweaking to get them working. With environment variable support, third-party component demos could easily reference (or be changed to reference) the Ogre sample media.