Updating the scripts to only execute when they are called directly.

Created by: GreenGremlin

In their current state, the scripts are cannot be imported without being immediately run. This change allows scripts to be imported and run directly. This change not only useful for creating custom script packages that wrap react-scripts, but is also a start toward making the scripts themselves testable.

const buildReact = require('react-scripts/scripts/build');

buildReact();

It would be nice to add the ability to pass options or custom config to the scripts, but I wanted to open this PR first and see where the discussion leads.