add valid / reactjs compatibility

[VinSpee]

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.

[benschwarz]

Sounds reasonable… but again, I'd really prefer to see this as a build option. @geelen, something for rework-am?