Add option to download chapters

maybe also hizb(half chapter)

Look at the existing CLI code to see how chapters are downloaded. The feature request is to add an option for downloading hizb (half chapter). Start by examining the argument parser and the download logic. Determine how to map hizb requests to the existing data structure and add a new command-line flag. Test the new option to ensure it downloads the correct content.

索引モデルが issue の本文から書いたものです。