title: onPositionChanged handler of gmaps marker scope issue
body: <p>I try to update the current position of a draggable marker of the react-google-maps wrapper and pass these values back to the parent.<br>
  My problem is, that in <code>onMarkerPositionChange</code> <code>this</code> is either the FindLocationMarker class or the marker itself, depending on how the function is invoked.<br>
  <code>onPositionChanged={() =&gt; this.onMarkerPositionChange()}</code> was my other option. </p>
  
  <p>I need both, the marker for the position and the class for the function in props.<br>
  In the provided example, <code>this</code> is the marker.
  <code>evt</code> is undefined, I know.  </p>
  
  <p>How can i provide both, the class scope and the marker, to the function?</p>
  
  <pre><code>class FindLocationMarker extends Component {
  
    onMarkerPositionChange = (evt) =&gt; {
      console.log(evt);
      let lat = this.position.lat();
      let lng = this.position.lng();
  
      this.props.handleMarkerPositionChange(lat, lng);
    }
  
    render() {
      return (
        &lt;div&gt;
          &lt;Marker
            position={this.props.position}
            draggable
            onPositionChanged={this.onMarkerPositionChange}
          /&gt;
        &lt;/div&gt;
        )
    }
  }
  
  export default FindLocationMarker;
  </code></pre>