=respond-to-width($width)
  $width : $width + 'px'
  @media screen and (max-width: $width)
    @content

=respond-to-width-beyond($width)
  $width : $width + 'px'
  @media screen and (min-width: $width)
    @content

=respond-to-height($height)
  $height : $height + 'px'
  @media screen and (max-height: $height)
    @content

=respond-to-width-or-height($width, $height)
  $width : $width + 'px'
  $height : $height + 'px'
  @media screen and (max-width: $width), screen and (max-height: $height)
    @content