Add create-react-wp-plugin and create-react-wp-theme packages

[rmccue]

We should make running this as easy as CRA itself. I want to be able to run:

npx create-react-wp-theme your-directory/
npx create-react-wp-plugin your-directory/

[roborourke]

So something I ran into is that the default webpack config in CRA uses the standard jsonpFunction name. I'm not sure it'd come up if not using code splitting but if someone is planning to use CRA for a plugin or theme things can break if config.output.jsonpFunction isn't namespaced.