Fix limitation regarding aria-expanded and aria-details

[lukewarlow]

As mentioned under limitations in the readme the polyfill doesn't handle the aria attributes. Ideally it should actually handle those, I think at least aria-expanded should be relatively simple to polyfill.

[keithamus]

I think at least aria-expanded should be relatively simple to polyfill.

The big issue will be in how aria-expanded is applied to buttons prior to the click handler, which it will need to be for popovers. This is not possible without possibly expensive scans of the document, MutationObservers, and other such machinery. Hard to know if it's well justified for those cases.