Rss Feed Filter

Would be awesome to have the possibilty to filter the rss feeds on arbitary fields.

Something like this
```yaml

- type: rss
title: News
style: horizontal-cards
feeds:
- url: https://feeds.bloomberg.com/markets/news.rss
title: Bloomberg
filters: # adding this Filter
- field: title
value: /Alert/i # RegEx or Just string?
- field: category
value: Business
- url: https://feeds.feedburner.com/crunchyroll/rss/anime?lang=deDE
title: Crunchyroll
filters:
- field: crunchyroll:seriesTitle # or just seriesTitle or extensions.seriesTitle ? Don't know how they get parsed by gofeed
value: # Here the values should be combined by OR because they would never match both
- /Failure Frame/i # if matched
- Berserk of Gluttony # Exact Value
```