title: Sass/Compass - Convert Hex, RGB, or Named Color to RGBA
body: <p>This may be Compass 101, but has anyone written a mixin which sets the alpha value of a color?  Ideally, I would like the mixin to take any form of color definition, and apply transparency:</p>
  
  <pre><code>@include set-alpha( red, 0.5 );          //prints rgba(255, 0, 0, 0.5);
  @include set-alpha( #ff0000, 0.5 );      //prints rgba(255, 0, 0, 0.5);
  @include set-alpha( rgb(255,0,0), 0.5 ); //prints rgba(255, 0, 0, 0.5);
  </code></pre>