Implementing `optionalAggregators` (#115)

Pull request for listed issue #115
I wonder if it's better to change this param to "activeAggregator" which be null or the 'id' of an aggregator. That would remove the need to have a "None" option in the menu. Making things a little bit cleaner.

However - we would also need to have developers give every aggregator a unique id.

What do you think?