support cue chunk encoding

[mattetti]

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

we should support the cue chunk

[mattetti]

Decoding is in place: https://github.com/go-audio/wav/blob/dcba81b6e09adf7ed74753c2c939c9f9ad277b61/cue_chunk.go

Encoding still missing