[Feature request] add audio support for vapoursynth combined video-audio

[sosie-js]

Hello,

We need to load the audio as well
-when loading video of a vpy script using the combination technique* as canal 1 audio of the video is ignored
-when loading audio of vpy script using the combination technique* as it is looking I assume for canal 0 (video)
instead of 1 (audio of the video)

*See at the end of the discussion
Thanks a lot for the vapoursynth bridge to aegisub, it's simply huge!