Add a mix tag to maps.

Add a mix tag only for map, example Mix=ConcreteTextures.

This will allow more customization for maps, like new terrain textures, different units, infantry, sounds, buildings, and much more things that would be made specifically for a custom map, all without actually effecting other maps, and default settings.
This may also help with maps that access certain .dlls, or other files, witch may cause recons.

The only downside I see form this feature is file sizes, longer loading with more mixes, and files, and people would need to have the custom mix files for those maps.