[vector-api] Configurable default styling for KML

This PR, at the request of @oterral, allows you to override the default style used for KML. Note that the [default default style](https://github.com/openlayers/ol3/blob/vector-api/src/ol/format/kmlformat.js#L140-219) is the same as Google Earth. In the default style you should be sure to set all styles (fill, stroke, image and text).

Example usage:
```javascript
var format = new ol.format.KML({
  defaultStyle: [new ol.style.Style({
    stroke: new ol.style.Stroke({
      color: 'red',
      width: 3
    })
    /* ... */
  })]
});
```