Feature Request : Pass custom parameters to FFMPEG

[Aymendje]

While the defaults are great, in some cases, being able to pass custom parameters would be really useful. For example, if we want to change default audio track, or add a subtitle track and merge them, etc.
Or even merging several mp4 into a single video.
This could be an "advanced options" menu that would would tick, to enable multi file uploading, and injecting custom options for ffmpeg (and other future conversions methods)

[C4illin]

there is now at least a env to always pass an arg to ffmpeg :)