Allow multiple priorities in the diggers

This would allow several items to be added to the priority of diggers. The digger would then prioritize digging items from the first added to the last. For example: If you're digging in a spot where Coal, Iron, Dirt, and Rock are all blended into a mess, you could add Coal, and Iron to the digger, and it would then focus on coal until there was none left to mine in the marked areas, then focus on iron.