add valid / reactjs compatibility

I'm currently working on a reactjs project where I'd like to use this module, but, unfortunately, react's html parser strips any non-valid attributes from templates. Also, it's probably best to offer people the option to write "valid" code if they so choose.

##Potential Impact
This does double the number of selectors, but the impact to a gzipped file is negligible, and this is a flex-box specific module, so the whole IE selector limit (IE ≤9) is a non-issue.