Add optional parameters "cause" (The entity that did the killing if available) and "force" (The force that did the killing if any) to LuaEntity::die()
As we know LuaEntity::die() triggers on_entity_died automatically, but it currently does not support passing the cause and force to the event, hence we don't know who triggered LuaEntity::die(). Adding them as optional parameters can solve this problem.
Edit: oops, turns out EvoGUI isn't using on_entity_died for the kill counter. So this is not related to the problem. But I will leave it here in case someone find it useful.
