Support decoding of inst chunks

[macbutch]

It looks like this would be quite easy to do following the same approach already used for smpl etc.

https://sites.google.com/site/musicgapi/technical-documents/wav-file-format#inst

There appears to be an error on that link as the chunk id is 'inst' not 'ltxt' (which is a text label). I could potentially start work o a PR if there's itnerest.