[IDEA] Detect and propose to include external subtitles

[astro-stan]

I like to include multiple subtitle streams in my media.

Let's suppose I have the following folder structure:

movie.mp4
movie.eng.srt
movie.fre.srt
movie.spa.srt

It would be nice if in the "Subtitles" section, FastFlix is able to detect that files with the same name as the input media, language code and subtitle extension are present and show them as options to include in the output stream. It would also be nice to have a button to include external subtitles in case they are in a different directory or have different name, that causes them to not be autodetected.