Allow for single section for all PRs

[morluna]

Our team is currently small and have opted to use emojis (https://gitmoji.dev/) in the PR title instead of using labels. It would be great to be able to to have a single section of release notes with all the PRs associated for that milestone.

Example:

Changed PRs:
- ✨ Awesome new feature [#37](https://github.com/....)
- 🐛 Fix bug [#38](https://github.com/....)

Unless I'm missing something, it seems that this isn't possible at the moment. It would be nice to define our config file like:

changelog:
  sections:
  - title: "Changed PRs"
    labels: ["*"]

[philwebb]

If you're interested in try to adapt the code you'd probably need to change ChangelogSection or ChangelogSections to collate things differently. If there's a way to keep existing compatibility, we'd be happy to review a pull-request.