Implement object subscripting

[acorscadden]

This framework looks pretty neat!

I think it could benefit from object subscripting. http://nshipster.com/object-subscripting/

[ahknight]

The problem is then finding a way to denote what type you expect back. Maybe self.theme.colors[@"someKey"] and self.theme.fonts[@"anotherKey"]?

In Swift those could be exposed as typed dicts to boot.