Allow for 4:3 Video embeds

@argyleink has requested a clean way to use the `Video` shortcode to embed with a 4:3 aspect ratio, instead of 16:9.

He's currently resorting to the following hack:

```

.adjusted-aspect-ratio {
aspect-ratio: 4/3;
}

{% Video
src="video/vS06HQ1YTsbMKSFTIPl2iogUQP73/rx8k6w5iQnEOwqOoUNNG.mp4",
autoplay="true",
loop="true",
muted="true",
class="adjusted-aspect-ratio"
%}
```