related to w3c/aria#1254

closes #221 - there were comments to do more with aria-hidden rules beyond just disallowing it on body, but i'm going to make a new issue for potentially covering those topics.

as `aria-hidden=false` doesn't seem to cause any problems, even per the note in ARIA to use with caution:
>At the time of this writing, aria-hidden="false" is known to work inconsistently in browsers. As future implementations improve, use caution and test thoroughly before relying on this approach.

let's limit this rule to just `aria-hidden=true`