adding aspect ratio param

[backface]

We find ourselves having a lot of archive (digitized) videos that are not in 16:9 but still should be responsive and full width.

Not sure if it's the most elegant solution, but I added parser that looks for a parameter like "4:3" or "16:9" and adds the matching aspect-ratio definition to the styles of the iframe (and unset the height, if there's an aspect ration definition)

I also added a coercion (back) to string to the condition in the sizeToStyle function as this was resulting in "width: 100%px"

let me know if you find this useful or think it needs further improvement.