Add per-element configuration via event data

Latency can be configured per-element by passing options when the event
listener is bound. If multiple event listeners are bound to the same
element, only the data from the first event listener will set the
configuration.

```js
// Configure latency to 650ms for #scrolling-div
$("#scrolling-div").on("scrollstop", {latency: 650}, function() { ... });
```

Fixes #4.