Checkbox warns when label is a partial.

I've got a use case where the label is an evaluation of some JSX.

ie:
`
label : ( <span>Accept the <a href="...">Terms and Services</a></span> )
`

I was going to publish a pull request to loosen the proptype, but don't necessarily disagree with a label being strictly a string.

Happy to do a pull request, is there a suggested way to allow checkboxes to accept small partials?

