Experimental first pass at UIViewController support

[davelyon]

I would like to make adding Cheat Codes as seamless as possible for existing apps, and require minimal additional work. To that end, this pull request is one possible solution to enabling any arbitrary viewcontroller to get help output and avoid having to directly use UIKeyCommand for debug work.

To opt in, your UIViewController subclass:

• Conforms to CheatCodeResponder

public protocol CheatCodeResponder: CustomDebugStringConvertible {
    var cheatCodes: [CheatCodeCommand] { get }
}

• Calls addCheatCodes() in viewDidLoad to have the UIKeyCommands registered behind the scenes.

In order for the help method to find all CheatCodes in the current responder chain, I'm using _deepestUnambiguousResponder on UIWindow to find the bottom of the responder chain, and then walking back upward myself to print out the chained commands.