Feature request: Optionally allow music playing in other apps to continue when BookPlayer audio starts

[fivestones]

This feature request is somewhat related to #570 as it is for the sake of having background music play when listening to an audiobook. However, the approach I'm suggesting is different.

There are a few apps which use audio, which also do not stop currently playing audio from other apps. For example, a workout app which gives you audio cues, but lets music you have play from spotify continue to play in the background. I've also seen metronome apps which do this--not stopping the music currently playing in the background while making the metronome sound.

What would be great is if there were a setting you could change in BookPlayer to either have the current behavior (stop other audio when BookPlayer start playing) or this new behavior (let other apps' audio continue when BookPlayer starts playing). Then I could go into Spotify or apple music or soundcloud or whatever, start some instrumental music, then go to BookPlayer, start a book, and have the music continue in the background.

Some sort of independent volume control for BookPlayer would probably be necessary, but other than that I think it might be easy to implement and a great way to listen to books with whatever background music you might want.