Hide empty cells

Adds support for a table level attribute [data-hide-empty] to let you
hide empty cells when in stacked view.
Added test for hidden cells

Added a test to make sure the cells are hidden/shown appropriately, not
sure how to ensure the test gets ran at a width under 640 px though.
Fix for automated build

Fix for a bug on the automated test environment for what I assume is due
to missing bracket
Merge branch 'master' of https://github.com/nickalbrecht/tablesaw into nickalbrecht-master

Conflicts:
	src/tables.stack-default-breakpoint.css
	src/tables.stack-mixin.scss
	src/tables.stack.js