This PR added support of much more STL container types in basic_node::get_value() API.
For details, please visit the documentation for the API.
Moreover, the API now accepts basic_node template instance types as its return type to allow conversions to container types whose element type is a basic_node template instance type, e.g., std::array<fkyaml::node, 3> or std::unordered_map<fkyaml::node, fkyaml::node>.
When this library is compiled with C++17 or greater, std::optional can be the return type which holds some converted value on success or std::nullopt otherwise.
Types which were convertible from nodes should still be convertible in the same way.

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• 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.