midifile + soundfont sidecar file

Hi,

For ease of use, perhaps we can support soundfont-files as sidecarfiles to midi-files, just like XMPlay?
Example:

```
mysong.mid    ---> check if mysong.sf2 exist
mysong.sf2     ---> exist! use this soundfont instead of the global soundfont
```

this makes it much easier to play midi-files made with custom soundfonts, without reconfiguring the global soundfont all the time.