47: Revamping QuickLook Playground APIs

Forum discussion: http://forums.swift.org/t/se-0198-playground-quicklook-api-revamp/9448

Proposal: https://github.com/apple/swift-evolution/blob/master/proposals/0198-playground-quicklook-api-revamp.md

Enabling Quick Look for Custom Types: https://developer.apple.com/library/content/documentation/IDEs/Conceptual/CustomClassDisplay_in_QuickLook/CH01-quick_look_for_custom_objects/CH01-quick_look_for_custom_objects.html

Playground Support library: https://developer.apple.com/documentation/playgroundsupport