lavfi: add compand audio filter

This patch adds the `compand` audio filter from ffmpeg master branch
(currently at 9026c49c82) adapted to work with libav.

The following changes are made from the ffmpeg compand filter:

 * use float instead of double
 * use strtok_r instead of av_strtok
 * remove asserts
 * use AVFrame instead of manually allocating memory
 * avoid av_uninit
 * use '|' instead of ' ' to separate parameters
af_compand: fix invalid read