Update JSON format to include default values and multiple releases

Update JSON format to include a property with default values, which can be any of the given parameters, and a property with all releases. Releases should be listed by the value (nominally the tag), and gitlab-release should accept an argument for the release to process. The output should be the original JSON formatted file for the one release (to be forward compatible with gitlab-org/release-cli#50 (closed)), and the release-cli command arguments. If a release with the argument is not found, this is not an error and the resulting output should be based only on the default values.

The original format should still be supported, although calling with a release argument and a single release file should be an error.