Add support for value prop

[ghost]

This component is really nice and beautiful, but unless I can add a value prop which is not between 0 and 1, it's quite unuseful for me. My use case is like this: I want a range between 0 and 500 and my value is somewhere inbetween (e.g. 235). When endTracking is fired I want to receive a value prop I can then use to store it. Could be something like that:

min={0}
max={500}
initialPosition={prevValue}
endTracking={(position, value) =>
    // do stuff
}