This is a common use case for me. I have TabNavigator inside StackNavigator and I want to set navigationOptions on tab navigator as If it is just a regular screen of stack. Current definitions won't let me do that, because NavigationTabRouterConfig limits navigationOptions. This problem is illustrated by lines added to test file.

This PR resolves this problem by loosening navigationOptions on tab and stack router. I took a liberty here and completely redefined NavigationScreenOptions type to be a union type, because 1) It's the best name for union type 2) Previous NavigationScreenOptions were just one-liner `{title?: string} and they're highly unlikely to be used on it's own, outside tab or stack navigator options.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

Select one of these and delete the others:

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes: <>
• Increase the version number in the header if appropriate.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }.