Refactor react-union-scripts structure

There are a couple of points that I'd like to address:

1. Rename the lib folder to something like utils.
2. Move the unionConfig handling code from utils to a separate package (or at least a separate folder or file).
3. Move webpack.config.js along with the parts to a separate package or a separate folder (or even a file, that would be sufficient).
4. I don't understand the point of having a separate startDevServer.js file, just merge it with the start.js file.
5. The scripts directory is a mess of multiple types of files. I'd like it to be a directory with the possible entry points for react-union-scripts and nothing else (test, build, start, that's it).
6. In my react-union-ssr-server package, I would like to use a couple of useful functions from react-union-scripts (namely the CLI getArgValue function), it makes me think whether we should have a react-union-utils or a react-union-cli package – more small packages will lead to looser coupling and more maintainable code.

The code itself is pretty good, but the directory structure needs a major revamp.

Start by mapping the lib, utils, scripts, unionConfig, webpack.config.js, startDevServer.js, and start.js paths in react-union-scripts. Review the existing entry points and the CLI getArgValue function, then define a coherent package or folder boundary for each concern. Done means the requested structure is implemented without losing the test, build, or start entry points.