Add --name-format and --direct options

[NonVideri]

The PR adds two options:

• --name-format sets the format for output file names. By default no format is set, and generic names from config.py are used. Supported values:
  • slug: Book title slug (e.g. "the-4-hour-workweek")
  • title: Book title (e.g. "The 4-Hour Workweek")
  • title-upper: Book title in uppercase (e.g. "THE 4-HOUR WORKWEEK")
  • id: Book ID (e.g. "617be9b56cee07000723559e")
• --direct saves files directly in the parent folder, instead of creating a new folder for the book. Requires --file-format to be set.