Player state variables to override untakeable/undroppable settings

[crankycyclops]

Related to #44: we should store some kind of list mechanism in a player's state to grant the player the ability to take "untakable" objects and to drop "undroppable" objects. The same should go in reverse, so we should be able to prevent a user from dropping an object that they could usually drop and from taking one that they could ordinarily take.

Not sure how I'm going to implement this yet. We'll see.

[crankycyclops]

Reopened on Trogdor-pp.