core/src/main/java/org/trimou/engine/MustacheEngineBuilder.java
public MustacheEngineBuilder registerCallback(
            EngineBuiltCallback callback) {
        checkArgumentNotNull(callback);
        checkNotBuilt();
        this.engineReadyCallbacks.add(callback);
        return this;
    }
public MustacheEngineBuilder registerCallback(
            EngineBuiltCallback callback) {
        checkArgumentNotNull(callback);
        checkNotBuilt();
        this.engineReadyCallbacks.add(callback);
        return this;
    }
Callback is useful to configure a component instantiated before the
engine is built.

@param callback
@return self