add "visually hidden" mixin/helper

[callmecavs]

add a class or Sass mixin to hide something in a manner that is still accessible to screen readers

html5bp uses:

.sr-only {
  border: 0;
  clip: rect(0, 0, 0, 0);
  height: 1px;
  margin: -1px;
  overflow: hidden;
  padding: 0;
  position: absolute;
  white-space: nowrap;
  width: 1px;
}