Feature/232 user defined literal for deserialization

[fktn-k]

User-defined string literals for YAML node objects have been implemented for input character sequences encoded in UTF-8, UTF-16 or UTF-32 format.
To follow the conventions in the standard library (like the literals declared in the \<chrono> header), the operator is declared in the namespace fkyaml::literals::yaml_literals, where both literals and yaml_literals are inline namespaces.
Access to the operator can be gained with:

• using namespace fkyaml::literals;,
• using namespace fkyaml::yaml_literals, or
• using namespace fkyaml::literals::yaml_literals

The newly added namespace literals is assumed to be used for other user-defined literals possibly implemented in the future.

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Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
• [x] The code coverage on your branch is 100%.
• [x] The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[coveralls]

coverage: 100.0%. remained the same when pulling dee621b191b8f73b2f254c67caf037dcc43a3811 on feature/232_user_defined_literal_for_deserialization into be5f6387ab57e47566ab035ccefb61b61903666a on develop.