convertDocument(documentEl) { // eslint-disable-line
    throw new Error('This method is abstract')
  }
Orchestrates conversion of a whole document.

This method should be overridden by custom importers to reflect the
structure of a custom HTML document or fragment, and to control where
things go to within the document.

@abstract
@param {DOMElement} documentEl the document element.

@example

 When a fragment `<h1>Foo</h1><p></Bar</p>` is imported the implementation
 looks like this.

```js
  convertDocument(els) {
    this.convertContainer(els, 'body')
  }
```

If a full document `<html><body><p>A</p><p>B</p></body></html>` is imported
you get the `<html>` element instead of a node array.

```js
  convertDocument(htmlEl) {
    var bodyEl = htmlEl.find('body')
    this.convertContainer(bodyEl.children, 'body')
  }
```