title: uber/react-map-gl getMap & exposed Mapbox API
body: <p>I'm using the following code snippet to try to access the MapBox API from uber  react-map-gl: 4.0.2 using mapbox-gl v0.50.0.  </p>
  
  <pre><code> import MapGL from 'react-map-gl';
   export default class App extends Component 
   {
     constructor(props) {
      super(props);
     this.mapRef= React.createRef();
     }
     componentDidMount() 
     {
        let data = this.mapRef.getMap().getBounds(); &lt;----
     } 
     render() {
      &lt;MapGL
      {...viewport}
      width="100%"
      height="100%"
      mapStyle={MapStyle}
      onViewportChange={this._updateViewport}
      ref={map =&gt; this.mapRef = map}
      mapboxApiAccessToken={TOKEN} &gt;
     }
   }
  </code></pre>
  
  <p>if I try to access any other methods like getStyle/getSource and others raise an error  "is not a function" &amp; "Cannot read property 'version' of undefined". Am i Doing something wrong or Not all MapBox Api methods are not available through the getMap() method ? </p>
  
  <p>Thanks</p>