Support MQs without selector

Useful if you want to share same styles accross different MQs, but some of those MQs don’t need scoped styles.

Example:
```scss
  @include scopedmediaquery(
    '(min-width : 30em)', '',
    '(min-width : 90em)', 'aside'
  ) {
  /* breakpoint styles here */
  .schedule-component {
      float: left; 
      width: 100%;
      position:relative; 
  }
  .schedule-component ul,
  .schedule-component li {
    list-style: none;
    position: absolute;
    margin: 0;
    padding: 0;
  }
}
```