Find a way to extend the Theme flow type

Say you want to add extra keys to your theme. Right now Flow will complain about them. Would  be nice to extend the theme type and make components be aware of it.

Example:

```js
const theme = {
  ...DarkTheme,
  colors: {
    ...DarkTheme.colors,
    primary: '#F44336',
    accent: '#F44336',
    secondaryText: 'rgba(255, 255, 255, .7)', // Custom one
  },
};
```