added ColorUtils.lerp(List<Color> colors, double t) function

[xonaman]

Similar to Color.lerp, but it supports more than just two colors.

Very useful in my case, so I thought I might as well share it.

Edit: Please add import 'dart:math' as math;, I forgot it

Example usage:

LinearProgressIndicator(
  value: progress,
  color: ColorUtils.lerp([
    Colors.red,
    Colors.yellow,
    Colors.green,
  ], progress),
)

[Ephenodrom]

@xonaman
Looks great! Can you please add a unit test ?