function alpha(color, value){
  color = color.rgba;
  if (value) {
    return rgba(
      new nodes.Unit(color.r),
      new nodes.Unit(color.g),
      new nodes.Unit(color.b),
      value
    );
  }
  return new nodes.Unit(color.a, '');
}
Return the alpha component of the given `color`,
or set the alpha component to the optional second `value` argument.

Examples:

   alpha(#fff)
   // => 1

   alpha(rgba(0,0,0,0.3))
   // => 0.3

   alpha(#fff, 0.5)
   // => rgba(255,255,255,0.5)

@param {RGBA|HSLA} color
@param {Unit} [value]
@return {Unit|RGBA}
@api public