Step durations for GitHub commit statuses

We've added step running durations to GitHub commit status step notifications.

When a step includes a :github_commit_status step notification the step running duration will be included in the commit status description on GitHub. The duration will show when the step is finished.

GitHub commit status step notifications can be configured in your pipeline.yml:

steps:
- command: build-the-thing.sh
  notify:
    github_commit_status: true