Aspect ratio improvement

@brianegan It would be cool to add one more improvement to Chewie. As we have autoinit - we can also add aspectRatio from controller value.
I see this as:
```
double _calculateAspectRatio(BuildContext context) {
final size = MediaQuery.of(context).size;
final width = size.width;
final height = size.height;

return _controller.value.initialized
? _controller.value.aspectRatio
: width > height ? width / height : height / width;
}
```

It can simplify video displaying in proper shape.
Waiting for your feedback
Thanks in advance