Add nodedef override/api

[Thomas Rudin]

-- "simple" move node
minetest.register_node("my:node", {
 on_movenode = function(from, to)
 end
})

-- callback for jump finalization stage (for example savefile writes)
jumpdrive.register_after_jump(function(from_area, to_area)
  -- from_area = { pos1, pos2 }
  -- to_area = { pos1, pos2 }
end)