#54 Unicode support

[Eugene]

Is it possible to add unicode support for path/filenames?

Current win32 build fails with "Error: Can't open the input file" when I run it in batch mode (optipng *.png) if there's unicode symbols (like ō) in path.

Optipng also fails with same error if theres unicode symbols in file names even in single file mode (optipng ō1.png).

But there's no problem if I run it for single image (optipng 1.png) even if there's unicode symbols in path.