add font initialize callback

[ousttrue]

Hello.

I've added flexible initialization with callbacks.
Register the function as follows for UnityEvent.

        io.Fonts.AddFontDefault();
        io.Fonts.AddFontFromFileTTF("C:/Windows/Fonts/MSGothic.ttc", 24, config.Ptr, io.Fonts.GetGlyphRangesJapanese());
        io.Fonts.Build();

This is useful.
Thank you.

[psydack]

Those changes will help a lot and I'll add the instructions into readme.

I really appreciate, thanks!