///
/// Generates `opacity` output for a given element and adds a filter for old IE.
///
/// @author bartveneman
///
/// @link http://caniuse.com/css-opacity caniuse
/// @link http://www.w3.org/TR/css3-color/#transparency spec
///
/// @param {Number} $value [0]
///
/// @output
/// ```css
///   opacity: <value>;
///    filter: alpha(opacity=<value * 100>);
/// ```
///
/// @example
///   .selector {
///     @include x-opacity(0.3);
///   }
///
@mixin x-opacity ($value: 0) {
	$value-percentage: $value * 100;

	opacity: $value;
	-ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=#{$value-percentage})";
	filter: alpha(opacity=#{$value-percentage});
}