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.

[NicoWeio]

Thanks for your pull request!
Rather than having explicit options, I'd like to accept a template, like how Instaloader does it. This way,

• We wouldn't need to add new logic to allow accessing of other (future) attributes
• Near-arbitrary directory structures become possible
• We wouldn't need to introduce a --direct flag; instead, the default template would contain the current behavior of creating subdirectories.

The only downside is that uppercase doesn't come for free; one would have to pass uppercase versions to the formatter separately or create a custom formatter. But I don't personally see the need for uppercase in the first place; maybe you have an example of a use case?
Do you agree with my suggestions? And would you be willing to implement them?

[NonVideri]

@NicoWeio I agree that the template would be a superior option. I am, however, still relatively new at Python, so I'm not confident in my ability to implement this more advanced version in a clean, Pythonic & efficient way, in a timely manner. (Please do a proper CR if you're thinking of merging this one!)

As for the uppercase, I added it for my particular use case — I have a knowledge base where I mark external notes with ALL CAPS FILENAMES to distinguish them from the internal ones.