Start method too strict for width and height parameters
One of the examples from http://www.w3.org/Graphics/SVG/IG/resources/svgprimer.html#SVG_web shows a use of viewboxes to stretch the svg to whatever its container is sized as, with width and height set to a percentage:

```
<svg xmlns="http://www.w3.org/2000/svg" width="100%" height="100%" viewBox="0 0 200 200">
<circle r="50" cx="100" cy="100"/>
</svg>
```

I made a few modifications to svgo to support this kind of Start:

```
// Startraw begins the SVG document with the width w and height h.
// Other attributes may be optionally added, for example viewbox or additional namespaces
// Standard Reference: http://www.w3.org/TR/SVG11/struct.html#SVGElement
func (svg *SVG) Startraw(w, h string, attrs ...string) {
    svg.printf(svginit, w, h)
    for _, v := range attrs {
        svg.printf("\n     %s", v)
    }
    svg.println(svgns)
}
```

Which is nearly identical to Start, but takes strings. svginit's width and height format verbs are changed to `%v` to accommodate. I'm not sure if this is a good fit for svgo, but some may find it useful.
