add browser scrollbar width detection

[seb3s]

I've added a simple function to detect the browser scrollbar width and adapt the style offset to this exact value.

[begrs]

Its definetely a rough work around to attach an element to the dom to check the width but ++ for cross browser fix that should work 100% of the time.
Withouth this fix, simple scrollbar does not work properly for mobile browsers with no permanent scrollbars or Safari 11.1. (even though the docu states it should)

[begrs]

My bad for missing this in my last review:
this does not work if the script is added before the dom is loaded.

but its easy to fix:
d.addEventListener('DOMContentLoaded', function () { ss.scrollWidth = getScrollWidth(); initAll(); }); ss.initEl = initEl; ss.initAll = initAll;