来源：https://superuser.com/questions/583393/how-to-extract-subtitle-from-video-using-ffmpeg
需要先通过 ffmpeg -i Movie.mkv 查看第几个 stream 是字幕，例如第6个 stream 是字幕，那么命令如下：
ffmpeg -i Movie.mkv -map 0:6 -c copy subs.srt
-i: Input file URL/path.
-map: Designate one or more input streams as a source for the output file.
s:0: Select the subtitle stream.