Added:
- AbstractFeed class which can be extended in custom feed implementations

Changed:
- Substitute laminas/laminas-feed and jdecool/jsonfeed with custom implementation
- Rename Enclosure class to Attachment
- Allow multiple attachments for JSON feeds
- Integrate FeedCategoryInterface into FeedInterface
- Integrate StyleSheetInterface into FeedInterface
- Adjust namespace for classes Attachment, Author, Category, Image

Additionally, the extension can now be used in non-Composer installations.

Preview release, feedback welcome.