Add player_interact_at_entity for Armor Stands

The new bukkit event is a little awkward. In testing I found PlayerInteractEntityEvent does not fire for Armor Stands. PlayerInteractAtEntityEvent fires for the same entities plus Armor Stands, even giving a click position for all entities. However, canceling a PlayerInteractAtEntityEvent only cancels Armor Stand interactions. 

As far as I know, there's no way to combine these two events because of the way they work, so I created another -- some of it even copied over from player_interact_entity. I'm not sure why it throws a "Cannot convert e to MCPlayerDropItemEvent" in the player_interact_entity event, but I left that in for now.

So this is that proposal to get the ball rolling. Alternatively we could name it player_interact_entity_at. I have some other thoughts, but I'd like to hear what you make of this first.
Except it's not a location. It's a vector. A relative position to the entity location. I think location would be misleading.