Support for <boost/std>::filesystem::path

Basically the same issue from the mailing list which is open since Boost 1.35: http://boost.2283326.n4.nabble.com/program-options-Problem-with-paths-that-have-spaces-td2576490.html

Problem: You cannot take a `*::filesystem::path` param that contains spaces.

Reason: BPO uses `lexical_casts` which uses stream extraction stopping on first whitespace

Possible solution: Special case for types having a ctor accepting a string and using that.

Related issue: https://github.com/boostorg/lexical_cast/issues/25 but won't be fixed there. Consider: `lexical_cast("\"/home/my user\"")` is considered OK but calling a program with `./foobar "\"/home/my user\""` is not OK. So this is a BPO issue

从 Boost.Program_options 的值解析开始，并阅读链接的邮件列表报告以及 lexical_cast issue #25，以了解所述行为。跟踪 filesystem::path 参数的转换方式，然后验证包含空格的路径能够被接受，同时不改变现有的命令行处理方式；payload 未指定要运行的源文件或测试。

由索引模型根据 Issue 内容生成。