Support embedding default font

[hyochan]

Is your feature request related to a problem? Please describe.
Support embedding default font to Typography would be wonderful.

Describe the solution you'd like
After loading fonts,

const [fontsLoaded] = useFonts({
  'RobotoSlab-Regular': require('../assets/fonts/RobotoSlab-Regular.ttf'),
});

Set default font like below should apply all Typography fontFamily.

setDefaultFontFamily('RobotoSlab-Regular');

Describe alternatives you've considered
N/A

[hyochan]

Closes via #138