feat: --destination option

[martin-minarik]

Resolves issue #23

1. Added "--destination" option to parser.
2. Added a "destination" parameter to the "download(...)"
   and "download_video_audio(...)" functions located in the "download.py".
3. Passed destination to "pytube.YouTube.download(...)" located in "download_video_audio(...)".

=)

[Flowrey]

Thanks for contributing, seems good to me, maybe you can just add a short flag -d and remove the named parameters in commands.download.

[Flowrey]

Thanks for the change, just one last thing the CI doesn't pass, you just need to run black and isort --profile=black on your code, and i will merge.

[Flowrey]

You forgot to run isort --profile=black . 😜

[martin-minarik]

I see that it quite depends on which working directory it's being called from.

I tried calling it from youtube_bz/commands and it seemed to work fine the first time. （°々°）