Automatic ratio

Instead of expecting container element to have fixed dimensions that match the content dimensions to maintain proportion, we could attempt to determine the appropriate ratio of the content automatically, e.g. by reading the image dimensions. This could be done real-time to accommodate changes post-initialization.
This might solve scaling issues in #7 when going full screen.
It might also be able to replace the `ratio` option when the zoom container is to have different proportion than the thumb container.