Load External subtitles

From easier document, pending to be updated.

Now support .srt, .ass and WebVTT format, local files only.

Requirements:

• Placed in the same folder as the media file
• Using the same file name as the media file

Media File: example.mp4 Subtitle File: example.vtt

• for multiple languages: valid language code following BCP-47 are required
  • For Chinese: example.zh.vtt, example.cn.vtt is invalid
  • For English: example.en.vtt
  • For other language, visit this list (opens in a new tab)