Feature request: external audio track autoloading

[eternal-sorrow]

Hi all.

Currently, mpv supports only explicit adding of external audio tracks using --audio-file commanline option. I think, it would be good idea to handle them automatically just like it's done with external subtitles, i.e.:

• --auido-auto commandline option, doing the same as --sub-auto
• --audio-paths option, analog --sub-paths

P.S. Thanks for great job you are doing!

[ChrisK2]

duplicate of #967.