Support copying subtitles

[OpenSourceAnarchist]

I think a great and easy change would be to support copying existing subtitles from the original video file to the resulting interpolated one.

I'm pretty sure this can be done by simply adding -c:s copy to line 31 and line 40 of inference_video.py, though I didn't want to make a pull request in case you would want to add additional checks in case subtitle copying failed for some reason. For example, not all container formats support embedded subtitles, though I know mkv does, and possibly mp4?

[2994dd42d75b0]

Use !mkvmerge --output outputwithsubs.mkv --no-video --subtitle-tracks 2 --sub-charset 2:UTF-8 '('input.mp4')' --no-audio '('rifeoutput.mp4')'