@nrwl/react-native: Add a template directory option for generating new apps

[bwilliamp]

I've been working with NX for over year now and I love the ease of use and everything that comes with it.

In my work though I have had to adjust the generator script to generate custom apps with things like single sign on, Intune, and App Center/ Code Push integration. I had to create a custom generator based on the React Native app generator and modify the path it gets the files for the app.

The problem with this is when a new version of Nx or the @nrwl/react-native packages gets updated I have to go back and make sure nothing has changed and adjust if it did.

With this change anyone could create their own template app with their own code and it would generate that when running the generate command below.

nx g @nrwl/react-native:app your-app-name --template=path/to/template