Subtitles by DEFAULT

Mux now supports setting the DEFAULT flag to "YES" on caption or subtitle tracks through playback modifiers that can be added to the manifest URL. This feature only affects manifests and works with all existing assets. Most players handle this automatically so long as autoplay is enabled and the track has DEFAULT set to "YES". Mux also accepts BCP-47 language codes for selecting a default subtitle language, trying to find the closest match if an exact match on the asset is not available.