Enable PropType checking of particular className props

[RylanH]

Often times a component will take a className as a prop which can override styling of the component. The component should be able to limit what can be overridden to guarantee the component works and acts in a particular way. Often times this means that only positional (top, left, right, margin, etc.) and some sizing styles can be applied. To enforce this, a component should be able to check the passed in className and see what styles would be applied.