add hierarchical document merge

[dayglojesus]

This is a feature request.

It would be highly valuable to have a simple means of merging multiple YAML documents hierarchically.
Example: yaml merge doc1.yaml doc2.yaml doc3.yaml

Wherein the first document would be given least precedence and the final/last doc, the greatest precedence. Adding this would provide users the facility to create ad hoc overlays using "partials".