Fix ffmpeg subtitle overlay

[Mashinow]

This update fixes the ffmpeg functionality and lets you add subtitles to your videos right away. I tested it on Windows 10, but I can't guarantee it'll work on other systems.

[what-the-diff]

PR Summary

• Improved File Handling in 'cli.py'
  The changes introduce a more robust way to handle filename without the extension. Originally, the base filename was grabbed but the methodology was not the most consistent. It now uses a specific function to ignore file extensions.

• Streamline Video Stream Variable Naming
  We've improved code consistency by renaming the video stream variable from 'video' to 'stream'. All code lines that previously referred to 'video' now correctly refer to 'stream'.

• Advanced Subtitles Addition to Video Stream
  Modifications have been made to the process of adding subtitles, making it easier and more efficient. By reshaping the code, it's now more direct and intuitive to add subtitles to the video stream.

• Updated Video Saving Process
  The previous way of saving the subtitled video has been switched out. The combination of ffmpeg.output() and ffmpeg.run() now helps to save these videos with specific required options. This change enhances the efficiency of the video saving process.