extractor: youtube: Set extension of AAC audio formats to m4a.

This, in particular, eases downloading both audio and videos in DASH formats
before muxing them, which alleviates the problem that I exposed on issue

Furthermore, one may argue that this is, indeed, the case for correctness's
sake.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
extractor: youtube: Fix extension of dash formats.

While we are at it, separate the audio formats from the video formats.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Merge pull request #1622 from rbrito/fix-extension

extractor: youtube: Set extension of AAC audio formats to m4a.