please add --gh-repo (combined owner and repo) #27

This would ease the use from GH actions, which does not have the "single" repo name in its default environment variables, only:

GITHUB_REPOSITORY The owner and repository name. For example, octocat/Hello-World.

While it is possible to split this combined one in a GH action before calling github-release it would be more useful to be able to pass that variable directly and let github-release split it (single place instead of many GH actions).