core/src/main/java/org/trimou/engine/MustacheEngineBuilder.java
public MustacheEngineBuilder registerHelper(String name, Helper helper) {
        return registerHelper(name, helper, false);
    }
public MustacheEngineBuilder registerHelper(String name, Helper helper) {
        return registerHelper(name, helper, false);
    }
Each helper must be registered with a unique name. If there are more
helpers registered with the same name an {@link IllegalArgumentException}
is thrown. Use {@link #registerHelper(String, Helper, boolean)} to
overwrite the helper.

@param name
@param helper
@return self