Feature Request: Support custom combination of music parts

Currently spleeter supports three flavours of separation:

* Vocals / accompaniment
* Vocals / drums / bass / other
* Vocals / drums / bass / piano / other

It would be really useful to be able to configure which specific parts to separate out.

For my use case, I want to be able to separate only the drums from the rest of the music, so after using spleeter I would get two parts: Vocals + instrumental w/o drums and the drum part.

Right now I can still achieve this with some manual work using the `4stems` parameter and using software to merge the vocals, bass, and other parts together.