Merge keys are specified here[1] for YAML 1.1. While not part of
the YAML 1.2 specification, they're very useful and are supported
in other implementations[2][3][4] that target 1.2.
Support for merge keys is optional and disabled by default. It can
be enabled by defining YAML_CPP_SUPPORT_MERGE_KEYS, either directly
or by setting the CMake option YAML_CPP_SUPPORT_MERGE_KEYS=ON.
[1]: http://yaml.org/type/merge.html
[2]: https://github.com/go-yaml/yaml
[3]: https://github.com/ruby/psych
[4]: https://bitbucket.org/ruamel/yaml
