Add `milestones` and `released_at` to release-cli params

Add the ability to specify milestones and released_at when creating a release. This capability exists in the REST API

Please see this related issue for more details. The changes in the releaser-cli should be coordinated with the changes in the :release yaml.

Implementation

• Add the milestones and released_at fields to the JSON request made to the Releases API by the GitLab client.
• Add flags --milestone and --released-at to the create command.

Requirements:

• --released-at Expected in ISO 8601 format (2019-03-15T08:00:00Z).
• --milestone array of strings