add essentia for audio and music analyses

[PeerHerholz]

@koudyk and I are going to start working on adding essentia functionality for audio and music analyses. Even though there's a python wrapper, the underlying installation and requirements seem to be kinda heavy. Hence, before we start, we wanted to get folks' (@adelavega, @tyarkoni) opinion about this, especially wrt using pliers as a python package or within the pliers docker image.