Pick react-script version to use in CLI

[joaosoares]

It'd be nice to have a way to optionally specify the react-scripts version to be used when creating a new app, so that we can try out the alpha versions on npm. Manually changing the version after the app is created won't copy over new template files such as the test folder. For example,

create-react-app --version="0.3.0-alpha"

[gaearon]

It is already possible via --scripts-version flag. It's not documented because we want people to use latest but it's there for power users. You can check the CLI source for it.

[joaosoares]

Thank you!