feature.toGeoJSON to return feature's ID

It would be convenient if the VectorTileFeature.toGeoJSON method returned feature's ID.

https://github.com/mapbox/vector-tile-js/blob/master/lib/vectortilefeature.js

``` javascript
...
return {
        _id: this._id, // proposal
        type: "Feature",
        geometry: {
            type: type,
            coordinates: coords
        },
        properties: this.properties
    };
...
```
