[Feature request] Add prop to customize third track color

[AlixH]

Expected Behavior

In the code there is a variable called trackThreeStyle set to unselectedStyle. I'd expect it to be set through a prop

Actual Behavior

Unable to get three different colors for the range slider (left, middle and right)

This has already been asked in #249 but I'd prefere not to use a fork as a solution (the solution seems pretty easy)

[manarfalah]

is there any progress on this issue ? I also want to be able to three different colors on two thumbs slider