Aria progressbar

Hi Aaron,
I saw your plugin on the WSG mailing list (Links for Light Reading) today and thought it was a great idea. I wanted to take it a step further and utilize the WAI-ARIA progressbar role (https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_progressbar_role).

I'm submitting a PR for your review. Please take a look and let me know if this is worthy of merging into your work. Thanks and please let me know if you have any questions or comments. Cheers!

William Dodson