v4: suggestion to introduce a color contrast function

Created by: bassjobsen

for instance: http://compass-style.org/reference/compass/utilities/color/contrast/#function-contrast-color

Useful in among others mixins/background-variants.scss:

// Contextual backgrounds

// [converter] $parent hack
@mixin bg-variant($parent, $color) {
  #{$parent} {
    color: contrast-color($color,$gray-dark,$gray-lightest);
    background-color: $color;
  }
  a#{$parent} {
    @include hover-focus {
      background-color: darken($color, 10%);
    }
  }
}