Ability to specify font package

[vanelizarov]

Hello!

My icon font is packed in separate lib and imported in main app, so I need to specify fontPackage param for every IconData for the font to work

Actually, there's a param in generated dart class, but it's hardcoded to null and every time I generate font I need to set it manually.

I know that it's kind of advanced use case, but it would be great to e.g. specify it manually once in config.json and forget 🙂