Support Unicode-filenames on Windows

[Fedr]

Currently libconfig functions take (const char *) arguments for file names and call fopen(...) function inside. This is perfectly right in Linux world where strings are stored in UTF-8, but unfortunately, this way does not support arbitrary non-ASCII characters in filenames on Windows. See http://utf8everywhere.org/ for details.

Could you please add some support for Windows as well. For example, if you accept filenames as std::filesystem::path, this will work perfectly. Or please implement any other method suggested in http://utf8everywhere.org/

[hyperrealm]

I don't have access to a Windows system at this time. If someone who does could create a patch to add this, I'd appreciate it.

[nenin-sc]

It is possible to use windows native wide string functions (_wfopen?) to get FILE handle and then use it in Config methods
void read (FILE * stream) void write (FILE * stream) const
Of course, it might have some portability footprint.