Placeholder SASS Mixin

_mixins.scss

/* 
Usage:
  $placeholder: #999;
  input {
    @include placeholder { color: $placeholder; }
  }
*/
@mixin placeholder {
  &::-webkit-input-placeholder { @content } /* webkit   */
  &:-moz-placeholder           { @content } /* ff 4-18  */
  &::-moz-placeholder          { @content } /* ff 19+   */
  &:-ms-input-placeholder      { @content } /* ie 10+   */
  &:-input-placeholder         { @content } /* other... */
}