Now we can use custom code to transform XML documents

In the fluent api we can provide Pipe<Node, T> to transform an xml to an
instance of an object using custom code (like JOOX), and to transform to
another xml (Pipe<Node, Node>). Example:


        final ProcessingFlow flow = Leviathan.flowBuilder().afterFetch()
                .sanitizeHTML()
                .toJavaObject(new Pipe<Node, Link>() {
                    @Override
                    public Link execute(final Node input) {
                        Match find = $(input).find("main_content")
                                .filter("#pfoo");
                        // ....
                        return new XXXX(....);
                    }
                })