Appendable performance

[Janning Vygen]

In DomContent the method render(Appendable writer) is already implemented and the method render() is abstract. This should be switched for more performance.

In ContainerTag for example the method looks like this:

 
    @Override
    public void render(Appendable writer) throws IOException {
        writer.append(renderOpenTag());
        if (children != null && !children.isEmpty()) {
            for (DomContent child : children) {
                child.render(writer);
            }
        }
        writer.append(renderCloseTag());
    }

But it would be much easier to implement it like this:

   @Override
    public void render(Appendable writer) throws IOException {
        renderOpenTag(writer);
        if (children != null && !children.isEmpty()) {
            for (DomContent child : children) {
                child.render(writer);
            }
        }
        renderCloseTag(writer);
    }

The difference is that in the first method each child is creating and returning a String object while the second method just appends to the same Appendable object in the rendering process.

Otherwise we are just glueing Strings together instead of taking advantage of a StringBuilder.

I will send a PR later on and will add some performance tests, too. Stay tuned...