Support for DOCTYPE declarations

Maybe not entirely necessary, but just for completeness the library should have a way of producing a DOCTYPE declaration.

e.g.
```xml
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
```

I may decide to work on this myself, but if you don't hear from me in a year, then I forgot about this.