Allow to disable cpal playback

[qarmin]

I use Rodio in my project to open and parse audio files to check if files are broken.
Due using cpal, my app crashes on OS which doesn't have installed audio packages and shows error

error while loading shared libraries: libasound.so.2: cannot open shared object file: No such file or directory

I don't need to play audio files, so it would be great to be able to disable cpal playback

[est31]

That's an interesting idea. PRs welcome to add an optional "playback" feature