minetest.register on craft

Description

 * Called when player crafts something
 * itemstack is the output
 * old_craft_grid contains the recipe (Note: the one in the inventory is cleared)
 * craft_inv is the inventory with the crafting grid
 * Returns either an ItemStack, to replace the output, or nil, to not modify it
 * IT DOESN'T WORK FOR FURNACE COOKING RECIPES!

Example
This sends in the chat when a player has crafted something.