Improve the individual slide container markup

[jasonwebb]

• [x] Eliminate role="tabpanel" since these don't really look like tabs, especially when multiple slides are visible at a time.
• [x] Remove aria-hidden="false" from all visible slides, since false is the default value and all this really does is make AT work harder for no reason.
• [x] Remove tabindex="-1", since slides should never receive focus anyway.
• [x] Add role="group" to each slide
• [x] Add aria-label="slide".
• [x] Add useGroupRole setting that can be used to turn off / on the role="group" and aria-label.
• [x] Add documentation about this feature change in the main README

[jacobraccuia]

Hi Jason, I am learning more about more accessible coding and have a question regarding aria-label="slide". Would it be better to have the ability to give each slide a name so it was more clear what the slide was depicting? When testing with my screen reader, although I gave a custom region label, there is no information when viewing this group and I need to dig deeper to learn about that slide.