make TemplateEngine configurable (see #211), reloaded

[decebals]

Implementation based on this comment.

I have a updated pippo-demo-template on pipe:

public class PebbleDemo {

    public static void main(String[] args) {
        // .peb is the default file extension
//        Pippo pippo = new Pippo(new TemplateApplication(new PebbleTemplateEngine(), "pebble/hello"));
        Pippo pippo = new Pippo(new TemplateApplication(new MyPebbleTemplateEngine(), "pebble/hello"));
        pippo.start();
    }

    public static class MyPebbleTemplateEngine extends PebbleTemplateEngine {

        @Override
        protected void init(PebbleEngine engine) {
            engine.addExtension(new AbstractExtension() {

                @Override
                public Map<String, Filter> getFilters() {
                    Map<String, Filter> filters = new HashMap<>();
                    filters.put("myupper", new MyUpperFilter());

                    return filters;
                }

            });
        }

    }

    public static class MyUpperFilter implements Filter {

        @Override
        public List<String> getArgumentNames() {
            return null;
        }

        @Override
        public Object apply(Object input, Map<String, Object> args){
            return (input != null) ? ((String) input).toUpperCase() : null;
        }

    }

}

[gitblit]

👍 Looks good.