Provide support of WaveExtensible sound format

According the specification of wav format, the WaveExtensible header should be used if [1]:
 - PCM data has more than 16 bits/sample.
 - The number of channels is more than 2.
 - The actual number of bits/sample is not equal to the container size.
 - The mapping from channels to speakers needs to be specified.

[1] http://www-mmsp.ece.mcgill.ca/documents/audioformats/wave/wave.html

Note that our writer contradicts the spec and write the usual wav_pcm header.