Add new parameter to download playlist or specific book

[sweep-ai]

Description

This PR adds a new feature to the safaribooks codebase that allows downloading a playlist or a specific book. The existing codebase only supports downloading a book based on its ID. With this new feature, users can now specify a playlist or a specific book to download.

Summary of Changes

• Added a new command-line argument --playlist to accept a playlist URL or a specific book ID.
• Modified the main function to handle the new argument. If the argument is a playlist, the code iterates over the books in the playlist and downloads each one. If the argument is a specific book, the code downloads that book.
• Modified the download function to handle the new types of input (playlist or specific book).

This PR enhances the functionality of the safaribooks codebase by allowing users to download playlists or specific books, providing more flexibility and convenience.

Fixes #4.

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To checkout this PR branch, run the following command in your terminal:

git checkout sweep/playlist-download

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