Adding a `viewbox` attribute to the generated svg.

Hi, I've bumped into a problem integrating the svg into a view: resizing the item made it cut its content instead of scaling it.

To fix it, I added a viewbox, like this:

```

```

This way the svg resizes with its content scaling.

I don't know why this hasn't already been suggested, but it helps a lot when styling.

I might suggest a PR if you're interested :)