The list of section identifiers and corresponding display names, and the
order in which they are rendered, was hard-coded. Some projects want to
customise this, so move it into the Config object so that it can now be
specified via config.yaml, e.g.
sections:
- [features, New Features]
- [issues, Known Issues]
- [upgrade, Upgrade Notes]
- [api, API Changes]
- [security, Security Issues]
- [fixes, Bug Fixes]
Change-Id: I914572c6a07ca81c54965b4b5a6b6aba50b3a787