A simple implementation for translating the generated subtitles using the DeepL API

[Akazfu]

A simple implementation for translating the generated subtitles using the DeepL API into any specified language, following the source code conventions.

[what-the-diff]

PR Summary

• Expanded README.md with Translation Instructions
  We've upgraded the README.md file to include a section on how to translate subtitles using DeepL API. This section also details setting up the 'DEEPL_AUTH_KEY' for authentication and usage of --translate_subtitles and --translation_language flags.

• Updated Command-line Interface (CLI)
  The cli.py file now includes the deepl import and supports the --translate_subtitles and --translation_language arguments. These updates make it easier to interact with our program from the command line and manage subtitle translations.

• Enhanced get_subtitles Function
  We've bolstered the get_subtitles feature to handle translations via DeepL API when the --translate_subtitles flag is active. The function now recovers the DeepL authentication key from the environment variables and employs it in the translate_text_with_deepl function.

• Added translate_text_with_deepl Function
  A new feature, translate_text_with_deepl, has been added to enable textual translations using the DeepL API.