Creates a GitHub release from a given tag.

Files:
  * An optional list of assets to upload with the release.

Arguments:
  * repository: the fully qualified repository name (user/repo).
  * tag:        the tag name to create the release for.
  * title:      an optional title for the release.
  * changelog:  an optional filename containing changelog data.
  * out_file:   an optional filename to write the resulting release URL to.
