Allow defining *highlightColor* as a function

Declaring the *highlightColor* as a function will provide more control over the color applied to each highlighted items.

 *highlightColor*, when defined as a function, is passed two parameters: series and datapoint and should return a color string. Example usage:

```javascript
highlightColor: function (series, datapoint) {
  return datapoint[0] < 5 ? "#f1493e" : "#ffce14";
}
```