support wstring and wchar_t?

[WillPanSUTD]

seems to be hard to extend to wstring and wchar_t file io.

trying to implement FileIO for multilanguage like Korean characters, and failed.

e.g. 한국어.obj

[syoyo]

If you are running code on MSVC or MinGW, you can do similar fix as done in TinyGLTF > syoyo/tinygltf#214

PR for unicode(utf-8) support in VC and MinGW are appreciated.

[AnyOldName3]

I noticed this while scrolling past and feel it's worth pointing out that there are still non-EOL versions of Windows that don't support UTF-8, so a TinyGLTF-style fix won't actually work everywhere.