This PR adds a new basic_node API get_value_inplace() which converts a basic_node value into a given type and writes the conversion result into a given output parameter.
Visit the documentation page for details.

Here are some brief notes for the get_value_inplace() API.

• all the currently supported STL container/scalar types by get_value() are also supported by get_value_inplace() since internal implementation for conversions is shared.
• not default constructible types can also be used
• 1D, 2D and 3D C-style arrays are supported (more than 3 dimentional arrays are not)

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.