Make visibility classes more automatic

[gakimball]

The Foundation for Apps visibility classes should mimic the ones in Foundation 5, which affect all breakpoints with a single class. For example, this is the current Apps implementation:

.show-for-medium {
  @include breakpoint(medium) {
    display: block !important;
  }
}

It should work like this:

.show-for-medium {
  display: none !important;

  @include breakpoint(medium) {
    display: block !important;
  }
}