Validate that current release is present in changelog

The `validate` command does not validate that the current release exists in the changelog (unless the `--rc` flag is used). Some of our publishing workflows depend upon the release existing, so we need this tool to help us ensure it's present.

The `validate` command already checks this correctly in `--rc` mode.

Acceptance criteria:
* When run without the `--rc` flag:
* When the release is 0.0.0:
* `auto-changelog validate` throws an error if the release is present in the changelog (0.0.0 should indicate that the package has not yet been released)
* When the release is anything other than 0.0.0"
* `auto-changelog validate` throws an error when the current release is missing
* We have unit tests for both cases