[Feature] Overlay (or merge) audios using ffmpeg within Colab Notebook, useful for creating guitar backtracks etc.

## Description

The current Colab notebook is great and it has two parts,

- Install spleeter
- Separate from command line

Another part called "Combine using ffmpeg" can be helpful.

If we use let's say 5 stem model, we can obtain 5 different wav files. One can just want to remove one file and keep the other four, for example, in the case of creating guitar backtracks, so we only need files without guitar and we need to combine the remaining four. It can be done with a very simple ffmpeg script, which I provided below.

`!ffmpeg -i bass.wav -i drums.wav -i vocals.wav -i piano.wav -filter_complex amix=inputs=4:duration=longest output.mp3
`
## Additional information

I know it is not a core feature of spleeter, as it is the job of ffmpeg, but it won't harm in my opinion to include an option to combine(merge, or overlay, since I am not the technical expert in audio field I don't know the correct usage of this word) the wav files that spleeter produces.