enable customization of handle color

[tannakaken]

I really like this library because it's simple and full-featured.
I added some simple customization to match the style of the app I'm creating.
I would be very happy if you could pull it.

[mnkhouri]

Hi @tannakaken , thank you for the PR.

I think this property would make sense within handle1, e.g.:

  handle1: {
    value: number;
    onChange?: (value: number) => void;
    color?: string
  };
  handle2?: {
    value: number;
    onChange: (value: number) => void;
    color?: string
  };

Could you please move it to there? I will then happily merge this PR