Cave support beams

There should be support beams made of wood logs (or planks) and fences that are generated in caves to break up the monotony of being underground for a long period of time.

On rare occasions they should generate with a crafting table just beside them and on even rarer occasions a chest with common mining items such as coal, cobblestone, sticks, or a half broken stone pick-axe. But not rare items like diamonds or golden apples.