aria2c support

[bablat]

Hello, and thanks for building this very specific tool I was searching for!

youtube-dl gives terrible performance for me with its single connection. I get less than 100kbps which makes this unusable.
The simple workaround I could think of was to use an external downloader. So I grabbed a statically linked aria2c and injected it into /usr/bin with a volume in my docker-compose setup.
Simple configuration parameters were then added to the configuration:
youtube_dl_args = ["--external-downloader", "aria2c", "--external-downloader-args","-c -j 16 -s 16 -x 16 -k 1M"]
and I can now reach ~1Mbps which is still slow, but much more reasonable. Suggest to perhaps include better support for external downloaders, at least in the documentation, and to consider including aria2c in the image itself.

Thanks again!

[bablat]

This isn't a real issue actually, should have posted this in the discussion area, I'll close this regardless.