Add Namespace to your target

Nearly two years ago you merged a [pull request](https://github.com/jbeder/yaml-cpp/pull/344) which added some support for *more modern* CMake.

A step toward the right direction I think, but unfortunately the CMake code now includes variables like `YAML_CPP_LIBRARIES` which leads to spaghetti code somehow...



It would be beneficial if you would export your targets instead, this is considered **best practice**, is there any good reason why you don't do it yet? If not, I can try to come up with a patch.



Have a look at https://pabloariasal.github.io/2018/02/19/its-time-to-do-cmake-right/#good-boys-export-their-targets



If ready, one could import the library and all necessary includes, flags,... via



```

find_package(yaml-cpp 0.6.0 REQUIRED)



target_link_libraries(${PROJECT_NAME}

    PRIVATE

        yaml-cpp::yaml-cpp

)

```

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