Add document.title

Wanted to play around with your codebase a bit; thought I should make myself useful.

Wasn't sure if I need to follow the [SVG instructions](https://html.spec.whatwg.org/multipage/dom.html#document.title), also node.namespaceURI was only on the root (not the children), so that needs to happen first anyway.
`node.type` is actually a direct mapping to [`GumboNodeType`](https://github.com/google/gumbo-parser/blob/2190f841e045014275bbec4784b8b1b6e6cb5af2/src/gumbo.h#L455-468), which (perhaps confusingly) doesn't always exactly match the DOM types. There's also a `"whitespace"` and a `"cdata"` type, which are still `Text` nodes as far as the DOM is concerned. Thus, this should be `if node.nodeName == "#text"` or `if node.nodeType == 3`. I should probably mention this in the documentation.