Request to support for ScreenBreakPoints 'num' Type

Hello!,
I would like to request you to support the ```num``` type . (Don't stick to the double.)

Model code
```dart
class _ScreenBreakPoints {
num desktop = 1024;
num tablet = 775;
num watch = 200;
}
```

Controller code
```dart
// getter
num get desktop_breakpoint => app.screenBreakpoints.desktop;
num get tablet_breakpoint => app.screenBreakpoints.tablet;
num get watch_breakpoint => app.screenBreakpoints.watch;

// constuctor
Controller() {
ResponsiveSizingConfig.instance.setCustomBreakpoints(ScreenBreakpoints(
desktop: desktop_breakpoint,
tablet: tablet_breakpoint,
watch: watch_breakpoint));
}

```

Thanks!