Hi Jan.
There is nothing very sophisticated that we can do with Spotify at this point. There is no plugin and I'm not aware of any methods for directly controlling Spotify from another program. The best you can do at the moment is to use mouse and keyboard emulation. Even there, it almost seems like Spotify designed the UI in a way that makes this difficult on purpose. For example, they have a key for toggling playback, but not one for play and one for pause.
If you search the forums, you will probably find some existing discussion of this topic.