The tool does not account for releases with no changes

[mcmire]

Currently the tool expects releases to consist only of sections. For instance.

## 0.3.0

### Added

- Some change

### Changed

- Another change

However, in rare cases we create releases that have no changes. Currently we've implemented it this way:

## 0.3.0

### Changed

- There are no changes in this release.

This is a bit weird, however, and it prevents us from being able to use the --pr-links option, as we then have to associate this entry with a PR. It would be great to be able to say:

## 0.3.0

There are no changes in this release.

To address this, we could either:

a) allow the body of a release to be either a set of paragraphs or a set of sections, OR
b) allow a set of paragraphs to be provided before any sections of a release