Added the ability to download books from playlists !

[amit-gp]

Hi folks ! Added the functionality to download books from entire playlists to the tool.

Resolves #344

• Added a new flag --playlist which will interpret <BOOK-ID> as <PLAYLIST-ID>
• Filters duplicates, videos, chapters, etc from playlists, and only downloads books
• Reuses existing flow to download single books
• Downloads books one at a time.
• Also fixed a render issue with the logo on some systems by making it a raw string

[Potential future improvements]

• Parallel downloads
• Better overall project structure, we could now look into modularising the code better
• Skip books already downloaded (Also helps with retries)

[Tested]

• Existing flows like downloading single books
• New flow with playlists + playlists with other media like videos
• Works with enterprise accounts

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I did not find a PR guideline / template, so do let me know if I can change anything !
Just used my best idea for integrating playlists with the flag approach.

Amit.

[amit-gp]

@lorenzodifuccia :)