add interaction toggle example

As discussed instead of calling adding/removing the interaction to/from the map we could add activate/deactivate methods to ol.interaction.Interaction (in ol3). But maybe we do not need that. Another option is to use setMap(null) on the interaction for the deactivation, and setMap(map) for the activation. This will not change the index/position of the interaction in the map's interactions collection. But I don't think this correctly works in ol3, so I would use addInteraction and removeInteraction for now.