Aria-hidden attribute incorrectly applied when more than one cell in view

When more than one cell is visible, only the currently selected cell is exempt from having an `aria-hidden: true` attribute affixed. This is non-compliant in instances where _more than one cell_ is in view.

The carousel would have to know which cells are inside/outside the viewport to correctly apply the attribute. @desandro called this out explicitly in this thread: https://github.com/metafizzy/flickity/pull/774#issuecomment-400517559
(which refs issue [638](https://github.com/metafizzy/flickity/issues/638))

**Test case:** https://codepen.io/jasonwbullok/pen/bGNOrpa
(e.g. inspect cell number two to see the `aria-hidden: true` attribute)

Start with the linked CodePen test case and inspect the carousel behavior when multiple cells are visible. Review the discussion in pull request #774 and issue #638 for the viewport-related context. Done means aria-hidden is applied correctly based on whether each cell is inside or outside the viewport, rather than exempting only the selected cell.

Written by the indexing model from the issue text.