Allow specifying DOCTYPE declaration (`<!DOCTYPE root ...>`) to write

I found no easy way of adding a DTD to the XML output. E.g.:

```
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE gsafeed PUBLIC "-//Google//DTD GSA Feeds//EN" "">
<gsafeed>
...
</gsafeed>
```

It would be nice to have a feature to add such elements.
