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

Examples:

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

   lightness(#00c, 80%)
   // => #99f

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