Remove support for persisting author-defined focus-ring classes

[kloots]

This PR addresses issue #35, removing support for persistence of instances of author-add focus-ring classes.

[marcysutton]

I'm a little confused about how to use this polyfill. Can you also update the documentation for the current usage?

[kloots]

@marcysutton you'd simply install it as a project dependency via npm and then include the built dist as part of your rollup.

[marcysutton]

Sorry, I meant the selectors necessary to use the polyfill. I got a bit turned around by the discussion regarding CSS classes and how they should be avoided because developers will grow attached to them, and I assumed at first this change would have an impact on usage.

In practicality, though, the .focus-ring class seems required until UAs actually implement :focus-ring.

[jonathantneal]

I think going further in the direction of attributes is actually better than classes. I will pitch a PR, and I won’t be offended if it is rejected.

[alice]

@marcysutton To try and clear up your understandable confusion:
This concerns an (admittedly poorly documented, that's on me) option to allow developer to opt in to always matching .focus-ring by simply adding a focus-ring class to any element which should always match .focus-ring.

This concept in general is something we need, and this seemed like a good option for the polyfill since we figured it would be something developer would attempt. It's an open question right now as to what the standards-based option for this might look like.