Compatibility with smooth scroll

Hello,

## Expected Behavior
It should work as expected/normal behaviour when using a smooth-scroll library, for example [locomotive-scroll](https://github.com/locomotivemtl/locomotive-scroll).

## Current Behavior
It does not work, which is likely because Lozad does not know the current scroll position.

## Possible Solution
Option to detect by native (existing) scroll or by another means on desktop? I.e...

```
smoothScroll.on('scroll', (obj) => {
updateScroll(obj.scroll.x, obj.scroll.y);
});
```

Here is an example of that: [Preview ](https://www.curtainsjs.com/examples/multiple-planes-scroll-effect-custom-scroll/index.html) / [Code](https://github.com/martinlaxenaire/curtainsjs/blob/master/examples/multiple-planes-scroll-effect-custom-scroll/js/multiple.planes.parallax.setup.js).

## Steps to Reproduce (for bugs)
Just install Locomotive Scroll, and Lozad as normal.

## Tested on:
Latest version of Chrome on Windows 10 and MacOS.