Add auto-changelog generation

[wagoodman]

#132 documented the release process. This issue is to capture the auto-changelog generation process into the release pipeline using this tool: https://github.com/github-changelog-generator/Github-Changelog-Generator/

The goal is to:

• generate a changelog from opt-in labels
• add the generate command to the release pipeline, replacing the goreleaser changelog with the generated one... something like: goreleaser ... --release-notes <(some_changelog_generator_cmd)

This may be something along the lines of:

 docker run -it --rm  -v "$(pwd)":/usr/local/src/your-app ferrarimarco/github-changelog-generator --user anchore --project auto-changelog-testing -t $GITHUB_TOKEN --no-pr-wo-labels --no-issues-wo-labels --future-release ${GIT_TAG} --unreleased-only