code: public MustacheEngineBuilder addMustacheListener(
              MustacheListener listener) {
          checkArgumentNotNull(listener);
          checkNotBuilt();
          this.mustacheListeners.add(listener);
          return this;
      }
docs: Adds a {@link Mustache} listener. Manually added listeners are always
  registered before listeners added via configuration extensions.
  
  @param listener
  @return self