Added implementation of Chrf metric

[fhieber]

Pull Request Checklist

• Changes are complete (if posting work-in-progress code, prefix your pull request title with '[WIP]'
  until you can check this box.
• Unit tests pass (pytest)
• System tests pass (pytest test/system)
• Passed code style checking (./pre-commit.sh or manual run of pylint & mypy)
• You have considered writing a test
• Updated major/minor version in sockeye/__init__.py. Major version bump if this is a backwards incompatible change.
• Updated CHANGELOG.md

[mjpost]

This looks good to me. It might be nice to integrate into sacreBLEU, but I don't think that's necessary now.