Add a release step that creates a GitHub release

[nicwortel]

This allows to list releases on GitHub Releases, which supports release notes, downloads, etc.

• Find a way to securely store the GitHub API credentials - probably in a file in the user's home directory. See for instance how Composer does this (~/.composer/auth.json)
• Create a new release using the GitHub API
  • Use the version number (without the VCS tag prefix) as the release name

Release notes, prereleases and release assets are out of scope for now and will be added as part of future issues.

Requires #6

Requires #4 to configure the repository name to create the release in - unless we can think of a smarter way to automatically determine this?