Add waveformat-extensible support in the wav muxer.

Patch (GSoC qualfication task) by Zhentan Feng spyfeng gmail com, see
"[FFmpeg-devel] Add waveformat extensible support in wav muxer (SoC
qualification task)" thread on mailinglist.

Originally committed as revision 18372 to svn://svn.ffmpeg.org/ffmpeg/trunk