Option to skip onSlide event on programmatic value change.

I have a use case where I do not want to fire the `onSlide` event if the value is programmatically set. For example, the following will trigger an `onSlide` event.

    $('input[type="range"]').val(10).change();

I would only like to fire that event when the user manipulates the input. I've added an option called `triggerOnValueSet` that will disable the event trigger if it is passed as `false`. The change is 100% backwards compatible.

Thanks for taking a look!