function saturation(color, value){
  if (value) {
    var hslaColor = color.hsla;
    return hsla(
      new nodes.Unit(hslaColor.h),
      value,
      new nodes.Unit(hslaColor.l),
      new nodes.Unit(hslaColor.a)
    )
  }
  return component(color, new nodes.String('saturation'));
}
Return the saturation component of the given `color`,
or set the saturation component to the optional second `value` argument.

Examples:

   saturation(#00c)
   // => 100%

   saturation(#00c, 50%)
   // => #339

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