Introduce nested categorisation

[mikepenz]

This new feature adds the ability to nest child categories in parent categories (as deep as needed)

A match in the child category will result in the item to no longer be shown in the parent.

A child match requires to also match the parent category.

It is now possible to define categories to consume matches, so a PR will not show up in another category after that.

• break out some functionality when transforming the pull requests into the changelog
• adjust logic to collect entries in the category (with the side effect for it to have automatic support to inject changelog lines)
• clear categories in tests as we run in the same process multiple times
• add new ability to recursively build changelog
• add feature to consume PRs if they were matched to a category
• keep header if children have entry