Add child-pipelines-auto-discover feature
Merges the child-pipelines auto-discover feature that was implemented but not merged before 0.18.0 was tagged.
This adds the child-pipelines-auto-discover option that was documented in the 0.18.0 ChangeLog but the code wasn't actually in the release.
This should be merged and 0.18.0 should be re-tagged (or create 0.18.1).