MKV container option

[hcook95]

Added an option to add soft subs to the video in a .mkv container.

[what-the-diff]

PR Summary

• Introduction of a New Parameter in Command Line Interface
  This upgrade introduces a new element named output_mkv in the command-line interface (CLI). This feature will give users the ability to save the subtitled video in their desired format, either an .mkv or .mp4 container. By default, the subtitled video will be in .mp4 format.

• Adjustments to Enhance output_mkv Functionality
  Specific conditional statements have been incorporated. These improvements guide the software on how to respond when output_mkv is set to True. In this case, it leverages on the ffmpeg.output instead of ffmpeg.concat method. Its objective is to enable smooth transfer of video and subtitle streams into an .mkv container.

• Flexible Output File Extension
  The output file extension has been modified for flexibility. With this, the output video can switch between "mkv" or "mp4" formats effortlessly. This is determined by the user's preference, specifically the value set for output_mkv.