Negative wheelSpeed not accepted

[rattai]

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Docs say negative values are allowed: http://ghusse.github.io/jQRangeSlider/options.html#wheelSpeedOption

This line blocks them: https://github.com/ghusse/jQRangeSlider/blob/ea1be22370525a32132c9b16c72252c5dfb9c6fa/jQRangeSliderBar.js#L99

The rest of the code looks like it'll handle negative values once the value > 0 check is removed.