Load subtitles using callbacks

Starting with VLC 3.0, you've add a very cool feature to create a media with custom callbacks to read the data from which can be used to create a media player. From what I've seen, "libvlc_video_set_subtitle_file" is the (only) way to add subtitles to media player.

It will be great if subtiles can be opened using callbacks too (maybe open them using a "libvlc_media_t" object ?).