feature / improving color-yiq function

[wtorsi]

Hi,
The function is very great (thanks), but it will be much more convenient if we add some arguments, like below.
What do you think?

@function color-yiq($color, $dark: $yiq-text-dark, $light: $yiq-text-light) {
  $r: red($color);
  $g: green($color);
  $b: blue($color);

  $yiq: (($r * 299) + ($g * 587) + ($b * 114)) / 1000;

  @if ($yiq >= $yiq-contrasted-threshold) {
	@return $dark;
  } @else {
	@return $light;
  }
}

[MartijnCuppens]

Great idea, can you make a PR for this @wtorsi?

[wtorsi]

@MartijnCuppens Yes, sure. But you need to fix your 'Contributing to Bootstrap' section.
Adding upstream master is not good idea for v4-dev.