Add make_latest option

[kdmiltner]

When generating a release, the one option that's missing for automating releases for my use cases is make_latest. Can this be added as an inputs option to update an existing release tag to be marked as the latest?

For example, if the tag_name I input already exists, and make_latest is set to true, the functionality would update that release to be marked as the latest.

[softprops]

I believe this should be fixed in #304