Add option to delete specific version

[jcansdale]

Add a simple filter to allow a specific version names to be deleted. This is a pale imitation of what we would like to do in #77!

I've manually tested it and it seems to work:

gpr delete jcansdale-test/maven-test/com.mycompany.app.my-app -k $GITHUB_TOKEN --version 0.111-SNAPSHOT --force

com.mycompany.app.my-app
  Deleting '0.111-SNAPSHOT'
Public package versions are not eligible for deletion. For more on our deletion policy, see https://docs.github.com/articles/about-github-package-registry/#deleting-a-package.

This is expected for public packages. ☝️

Fixes #86