fix audio conversion not working with newer versions of ffmpeg

[nerdinand]

I tried to run the following command with the newest youtube-dl version and ffmpeg 1.1 installed and got an error:

$ youtube-dl -x -k "https://www.youtube.com/playlist?list=PLr9rc7slvdRXfdRUITmYArOtB-99mr5h5"
[youtube] PL r9rc7slvdRXfdRUITmYArOtB-99mr5h5: Downloading page #1
[youtube] PL r9rc7slvdRXfdRUITmYArOtB-99mr5h5: Found 25 videos
[youtube] Setting language
[youtube] ytKvgLuy7ng: Downloading video webpage
[youtube] ytKvgLuy7ng: Downloading video info webpage
[youtube] ytKvgLuy7ng: Extracting video information
[download] Destination: ytKvgLuy7ng.mp4
[download] 100.0% of 82.40M at 1.33M/s ETA 00:00
[ffmpeg] Destination: ytKvgLuy7ng.aac
ERROR: audio conversion failed: Error splitting the argument list: Option not found

The error occurs because in newer versions, ffmpeg doesn't accept the -- option any more:
$ ffmpeg -y -i ytKvgLuy7ng.mp4 -vn -acodec copy -f adts -- ytKvgLuy7ng.aac
Unrecognized option '-'.
Error splitting the argument list: Option not found

So I did some poking around in the code and found a solution which on my system works with at least versions 0.11.1 and 1.1 of ffmpeg.

[phihag]

How else will filenames starting with a - be distinguished then? This seems to be a bug in ffmpeg; using -- to separate options and filenames is wide-spread convention.