Include support for reading of WavPack MD5 signature of raw audio data

Like FLAC, WavPack can also compute & store the MD5 signature of raw audio data when encoded using ```-m```

It can be displayed using ```wvunpack -f7 filename.wv```

Please add support to retrieve the md5 from WavPack files when present.

I'd be happy to provide a link to a downloadable file if required.