Lighting support #60

Lighting will need to be calculated each time a block is updated.

For reference:

• https://minecraft.gamepedia.com/Light
• https://en.wikipedia.org/wiki/Flood_fill

Ideally, this would be done multithreaded on a per-chunk basis.

I'm not sure how to correctly handle block updates. We could recalculate the light for the entire chunk, but this is incredibly inefficient.