Addition of a .fromDom constructor.

[NamanShergill]

Use case

Currently, the library only takes the HTML input as string, and parses it as a document internally. This adds unnecessary overhead if you already parse the document in your app to perform operations on it, so it has to be converted into a string again to be passed to the widget, and then the widget parses it back into a document. By adding a HtmlWidget.fromDom(Document document), this overhead could be eliminated.

[daohoangson]

I think this would be a great enhancement.