Added support for a data attribute to hide empty cells

I added support to hide cells that are empty from the stacked view if the attribute [data-hide-empty] is present on the parent table, along with a demo page that is modified from the stack demo. I also added a test after I found the contribution document asks for one (though I'm not sure how to force the viewport of the test environment to run under 640 to ensure it's hidden in an automated test environment). Hiding the empty cells also helps to clean up the footer when combined with the recently added option to hide labels for certain rows.