Implement React.PropTypes.exactly

[chicoxyzzy]

I took use case below from some gitter chat where @JustBlackBird wanted some property to be String or exactly false

Use case

PropTypes.oneOfType([
  PropTypes.string,
  PropTypes.exactly(false)
]);

instead of

PropTypes.oneOfType([
  PropTypes.string,
  PropTypes.oneOf([false])
]);

I can implement it if it does make sense.

[chicoxyzzy]

moved to #6166