Missing out-of-box support for some audio formats

Hi.

With default setup (and vlc backend) bongo is unable to play `m4a` and `opus` files which are not extremely rare.
I was able to workaround it via adding these format to lists of extensions and adding
```
(setq bongo-custom-backend-matchers
            '((vlc local-file "m4a")
              (vlc local-file "opus") ))
```
to my `.emacs` but in my opinion it should not be necessary.