Add the ability to have "knobs" to set props on React example that live outside of the example itself.

[mcarrano]

Want to move controls that modify an example outside of the actual example so they are not confused as being part of the example. See https://docs.google.com/presentation/d/1K-VBEwKtTMVnwvRxrjvSBOx-9jsdaAubm9V2ZeOt-sI/edit#slide=id.g12cf927eb17_0_0 [item 4] for what this might look like.