Draft release notes cannot be deleted

Here is the use-case I am trying to solve:

I'd like to tag my repo with the tag `latest` and draft a release with that tag every time PRs are merged into `master`. When that code is actually deployed to production, I create a tag using the git sha as the tag name and create a new release:

```
gren release --ignore-tags-with=latest
```

This creates a new release with the newly created tag. However, it also leaves a draft release for the `latest` tag even though the new tag is at the same level as `latest`. Until the `latest` tag moves ahead of the new tag, it's release notes are a duplicate and in the draft state.

I would like the option to remove a draft release using `gren` and am happy to submit a PR. The proposed command options would be: `gren release --delete-tag=`