support for local files as input videos

[micheleAlberto]

fixes #25

BEFORE: the video cli argument must be a youtube url AFTER: the video cli argument must be a youtube url or local file

[what-the-diff[bot]]

• Added type annotations to the function parameters
• Changed get_audio() so that it can accept local video files as well as URLs
• Fixed a bug where audio was not being extracted from videos when using ffmpeg (the command is now run)

[sloganking]

Is this meaningfully different from #28 ?

[ahmed024]

Yes, there is different this and #28 . With this, you dont get the this error ModuleNotFoundError: No module named 'utils'

[micheleAlberto]

A friend would like to use this. Can we merge it or should I tell him to work from my fork?