Added support for Google Maps custom styles

Added the `mapOptions` object to the layer options. This object contains the set of options to forward to the Google Maps constructor.

At the moment, two things are customizable:

* `styles` accepts an array of [`MapTypeStyle`](https://developers.google.com/maps/documentation/javascript/reference#MapTypeStyle) objects to customize the map style;
* `backgroundColor` to display while the map is loading.

Reference: https://developers.google.com/maps/documentation/javascript/styling