Support for more than one video on single page

added feature without breaking API.
added param for custom target DOM element.

```html
<script src="path/to/youtube.ga.min.js"></script>
<script>
    var configYouTubePlayer = [{
        videoID: 'Rk6_hdRtJOE', 
        element: 'player1'      // DOM element is required for multi video tracking
    },{
        videoID: 'Rk6_hdRtJOE', 
        element: 'player2'
    },{
        videoID: 'Rk6_hdRtJOE', 
        element: 'player3'
    }];  
</script>
```