feat: add categories item/entry filter.

[rdvn]

This pr introduces optional categories field for item/entry and keep_only / discard categories field filter. Categories are quite practical for e.g. lobste.rs rss feeds to follow only specific topics of interest.

Example config:

- path: /lobste.rss
    source: https://lobste.rs/rss
    filters:
      - keep_only:
          field: categories
          matches:
            - osdev
            - cryptography
      - limit: 5
      - full_text:
          simplify: true

Not sure what the contribution policy is, so feel free to reject. I suspect this isn't just enough to be merged as is and I'm ready to contribute the missing pieces. And thank you for this delightful piece of software.

[shouya]

That's a nice feature to have, thanks for the contribution!