Add support for Placemark inline Style

Hey Tom!

`Placemark` elements can have a `Style` child, has defined in the [schema](http://schemas.opengis.net/kml/2.2.0/ogckml22.xsd). This PR add support for both [LineStyle](https://developers.google.com/kml/documentation/kmlreference?hl=fr#linestyle) and [PolyStyle](https://developers.google.com/kml/documentation/kmlreference?hl=fr#polystyle).

I've also added a dedicated test case.

Thanks for reviewing :)

Yohan