Modularise scripts

Created by: djgrant

Would a PR to modularise scripts in react-scripts be considered?

For example, with start.js:

scripts
    start.js
utils
    addWebpackMiddleware.js
    createWebpackCompiler.js
    createWebpackDevServer.js
    runWebpackDevServer.js

Reasons for doing this:

1. Clearer for new contributors and people reading code for learning
2. Make the process of modifying an ejected CRA app simpler
3. Make scripts more extensible in ejected apps and forks (by removing interweaving and enabling reuse)
4. Reduce complexity of upstream merges to forks of create-react-app (my motivation)