saturate($color, $amount) 
Makes a color more saturated. Takes a color and a number between 0% and 100%, and returns a color with the saturation increased by that amount.
  Examples: saturate(hsl(120, 30%, 90%), 20%) => hsl(120, 50%, 90%)
saturate(#855, 20%) => #9e3f3f       Parameters:  $color (Color)   $amount (Number) —  The amount to increase the saturation by, between 0% and 100%    Returns:  (Color)  Raises:  (ArgumentError) —  if $amount is out of bounds, or either parameter is th