Add support for std::wstring (wide string support for Windows)

[MaximAshin]

When defining an option with Value<std::wstring> it won't compile (opposed to std::string).
Seems like there are some operators that are in use with std::string that are non-existant in std::wstring.

This support is especially important with command line params that hold Unicode values, like paths with Unicode characters.

Would really appreciate it if you could add support :)
Thanks in advance!