Allow width and height to be passed in as strings.

[poteat]

The type and value system doesn't allow widths or heights passed in as 100%, which is useful for resizing the SVG viewing component.

You can force it, and it works well, but it results in a warning / error being logged to the console, which is not great.

There are other approaches, but they involve re-renders, and I really want to just use CSS for this instead of full React state.