Allow wav2json to be compiled to read wav data as float instead of short

This changes https://github.com/beschulz/wav2json/blob/master/src/wav2json.cpp#L99-L100 so that this program can be used on 32-bit floating point wav files, as well as the other encodings.

Without this change, `wav2json` outputs an array of all zeroes on unsupported .wav types, causing unpleasant surprises.

I'd be happy to discuss the implementation details further - this works for our use case, but it's certainly not the only way to do it.