Tweak emitter

According to the Visitor Design Pattern, a visitor implementation could be provided or extended.

The following example disables javascript scripting. 

public class NoJavascriptHtmlEmitter extends HtmlEmitter {
    public NoJavascriptHtmlEmitter(Appendable buffer) {
        super(buffer);
    }

    protected boolean isTagAllowed(Tag t) {
        return !t.getName().equals("script");
    }

    protected boolean isTagAttributeAllowed(TagAttribute a) {
        return !a.getName().startsWith("on");
    }

    @Override
    public void visit(Tag node) {
        if(isTagAllowed(node)){
            super.visit(node);
        }
    }

    @Override
    public void visit(TagAttribute node) {
        if(isTagAttributeAllowed(node)){
            super.visit(node);
        }
    }
}