Add Accessiblity Classes

Add Accessibility Classes. `.hidden` hides element but allows screenreaders to read. Do not confuse with `.hide`. `.visible` reverses effect of `.hidden`. Less confusion with `.show`.

See https://github.com/zurb/foundation/issues/3597 for some background information.

Many thanks to @mmikitka for starting to improve accessibility and for defining the mixin used here. The rules within that mixin come from here: http://snook.ca/archives/html_and_css/hiding-content-for-accessibility