Get label positions

First I want to thanks for this plugin cause it helped me a lot for a project.

## The problem : HTML labels

I read some issues where people want HTML tooltip and I have the same problem but I think it's out of the scope of this plugin. A callback or a way to get label positions could help solving this issue.

## First Idea

Expose the "labels" offset position, something more accessible than :

```
chart.$datalabels._labels[1]._el.getCenterPoint()
```

## Second Idea

Maybe a third parameter to the formatter that would give the anchor position (x, y) so it could be used as a starting point to build custom HTML label (with absolute positioning it could work)

```
datalabels: {
anchor: 'end',
align: 'top',
offset: 2,
backgroundColor: '#FFF',
borderRadius: 4,
formatter: function (data, chart, anchorposition) {

}
}
```

But it won't solve the resize problem :(