Preserve flexible attributes across re-import

### Use case

So, sometimes you import songs or albums that you already have in your library. When you replace your old files with the new ones, you'll lose data in flexible attributes. But that data might be of some value for you. I'm thinking of `play_count`, `skip_count`, or my own `speellijst`.

### Solution

Create an option (`save_fields`?) to define which attributes should survive such event. I guess it should only work for songs with the same `mb_trackid`.