Alternate backend

[MrinalJain17]

Backend changes

• Scikit-video is no longer being used as the backend. The new backend is now ffmpeg-python -> Resolves #13
  • Related dependencies have been removed (Pillow)

API changes

• Removed required_fps parameter, and in order to simplify the API, merged the functionality of mode and extract_position
• Initial support for custom frame selection -> Resolves #6

Code factoring

• Added type annotations
• Refactored code

Documentation

• Updated docs in accordance with the changes in the backend and API

Version bump to 2.0.0