This PR has fixed the issue reported in #428, that is, conversion failure from a string to either an integer or a floating point value throws a parse_error exception.
The error should be recovered by treating the source scalar as a string scalar instead if the scalar is not tagged with !!int, !!float or the equivalents.
Note that conversion to null and boolean scalars are not considered as recoverable since their presentations are restricted to literals defined in the YAML spec, e.g., null, true or False.

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.