Add example link to README
Explain latency configuration
Use a codeblock for JS highlighting
Correct indentation of scrollstart function body

The lower 1/3 was indented one level too far.
Wrap comment since it's too wide for GitHub's view
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.