Dirty fix for opus format.

The opus format require some special execution. The usual one with
avconv/ffmpeg is not enough. The easiest way to do it is by converting
to wav with avconv/ffmpeg and then using opusenc to convert it to opus.
opusenc is part of the opus-tools package.

This should solve the problem in Issue 1263 - https://github.com/rg3/youtube-dl/issues/1263