Caption Subtitle Support

Now support .srt and WebVTT format

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 or example.srt

• for single language: filename should be identical with media filename
  • example.vtt is allowed, not example.en.vtt
• for multiple languages: valid language code suffix are required
  • For Chinese: example.zh.vtt, example.cn.vtt is invalid
  • For English: example.en.vtt
  • For other language, visit this list