Allow startUnicode to start at 0xf0000
I would like to have the `startUnicode` value be `0xf0000` but it does not seem to work. From what I can tell it is because `String.fromCharCode` used in `writeFontStream` only works up to `0xffff`. Is it possible to support 5 digit unicode characters (such as using `String.fromCodePoint` instead?) or is there perhaps some workaround I can use in the meantime?