// center vertically and/or horizontally an absolute positioned element
@mixin center($xy:xy) {
    @if $xy == xy {
        left: 50%;
        top: 50%;
        bottom: auto;
        right: auto;
        @include transform(translateX(-50%) translateY(-50%));
    }
    @else if $xy == x {
        left: 50%;
        right: auto;
        @include transform(translateX(-50%));
    }
    @else if $xy == y {
        top: 50%;
        bottom: auto;
        @include transform(translateY(-50%));
    }
}