[Feature Request] Add option to pick cover art source

With Airsonic-Advanced, we added an option to configure the source of cover art, which included the following choices:

- File only
- Tag only
- Prefer file over tag
- Prefer tag over file

It would be great to see these options available in gonic as well.

https://github.com/airsonic-advanced/airsonic-advanced/pull/317
https://github.com/airsonic-advanced/airsonic-advanced/pull/345