Enhance "!include" for YAML.

• Add ability to override properties and nodes by supplying them under the !include statement. (Similar to how one can supply properties and node with "extends" in JCR)
• Supports override keyword to stop the extending.
• Introduce a new no-space syntax to be valid YAML: !include:/light-yaml-sample/samples/sampleTab.yaml
• Deprecate (but still support) the existing "space-syntax" and print warning (if possible) when detected (However, override is not supported on the space-syntax): !include light-yaml-sample/samples/sampleTab.yaml

Note: this is a part of the "Inheritance of definitions from registry" initiative (former "YAML extends").
https://wiki.magnolia-cms.com/display/PMTEAM/YAML+extends+LDV
After discussion it was decided not to introduce the term "extends" for YAML to prevent confusion between how the feature functions in JCR.