Sass media query mixin that accepts media query string as argument

_mq.scss

/*-- Media query variables --*/

$mq-small: 'media and (max-width: 767px)';
$mq-large: 'media and (min-width: 768px)';

/*-- Media query mixin --*/

@mixin mq($mqString) {
    @media #{$mqString} {
        @content;
    }
}

// Usage: @include mq($mq-large) { /* CSS rules */ }