Add box-fade

[kjbrum]

@mixin box-fade($side: both, $color: #000000, $size: 100px) {
    position: relative;
    z-index: -1;

    $selector: '&:before, &:after';
    $top: 0;
    @if $side == top {
        $selector: '&:before';
    } @else if $side == bottom {
        $selector: '&:after';
        $top: 100%;
    }

    #{$selector} {
        content: '';
        position: absolute;
        top: $top;
        left: 0;
        width: 100%;
        height: 0;
        box-shadow: 0 0 $size $size*0.75 #{$color};
    }

    @if $side == both {
        &:after {
            top: 100%;
        }
    }
}