Allow Nodes to be removed in NodeTraversor

[jhy]

This change allows nodes to be removed during the head() method of NodeVisitor. E.g.

        Document doc = Jsoup.parse("<div><p id=1>Zero<p id=1>One<p id=2>Two<p>Three</div>");
        NodeTraversor.traverse((node, depth) -> {
            if (node.attr("id").equals("1"))
                node.remove();
        }, doc);

Implemented with 2bad736