add renderTrackLabel prop to render a label below/inside/above the track

I imagine the rendering could be above/below or even inside the track itself (the number inside the rectangle). so many possibilities but basically user could do: `renderTrackLabel={({ value, editing, ... }) => editing ? {value} : null}` (x would be set by a parent `` but as you would be probably centered, you would have to offset y by a given amount)