/**
 * @method 透明层opacity 
 * @description 透明层 内容也透明
 * @param {number} $opacity - 透明度 0 ~ 1之间
 * @example 调用
 * 
 * ```scss
 * div {
 *    @ include opacity(.5);
 * }
 * ```
 * 
 * ```css
 * div {
	  opacity: .5;
	  filter: alpha(opacity=50);
 * }
 * ```
 */

@mixin opacity($opacity) {
  opacity: $opacity;
  // IE8 filter
  $opacity-ie: ($opacity * 100);
  filter: alpha(opacity=$opacity-ie);
}