feat: support adding no subtitles

[EKarton]

Description:

This PR contains multiple changes:

1. Adding no subtitles to the generated PDF
2. Creating integ tests for the main.py file
3. Refactoring the test code

Usage of (1)

To add no subtitles to the generated PDF, one would need to supply the -S or --skip-subtitles flag to the cli tool, like:

python3 -m src.main tests/videos/input_1.mp4 -S -o output.pdf   

Running the command above will output a pdf with no subtitles, like:

output.pdf