VTT Output support added

[zaltinsoy]

Added an option to create subtitle files in the .VTT format.

[what-the-diff]

PR Summary

• Implementation of a new command line flag for adjusting subtitle format
  A new command line flag --subtitle_format, has been introduced. This will help users specify the subtitle file format type they prefer, whether it be srt or vtt.

• Modification of the 'get_subtitles' and 'write_srt' functions in 'cli.py'
  The functionality of these two functions has been updated to support multiple subtitle formats. The format handled by these functions will now depend on the subtitle_format argument provided by the user.

• Modification of 'format_timestamp' and 'write_srt' functions in 'utils.py'
  Similarly, these functions have also been updated to facilitate different subtitle formats according to the subtitle_format argument.