/// Container for vertically centering child elements.
/// To comply, child elements must have `vertical-align: middle;` or you
/// can simply `@extend %o-v-center-child`.
/// Note that `%o-center-parent` also has `vertical-align: middle`, so parents
/// can be nested.
/// @see %o-v-center-child
/// @link http://codepen.io/Lokua/pen/zxKQWG?editors=110
%o-v-center-parent {
  display: table-cell;
  vertical-align: middle;
}