Integrate react-app-tools

What do you think about extracting (1) `webpack.config.js`, (2) all the dev dependencies (~50), and automation scripts (`build`, `run`) into a stand-alone npm package ([`react-app`](https://github.com/kriasoft/react-app))?

This way it might be easier for you to keep a project based on RSB template up to date, you just update `react-app-tools` dev dependency in your project from time to time, which will ensure that you're using the latest and greatest configs and CLI tools ready to be used for building, running and testing your React application.

The `package.json` file in your project will look nice and clean, literally:

``` json
{
"private": true,
"dependencies": {
"history": "^3.0.0",
"react": "^15.2.1",
"react-app": "^1.1.1",
"react-dom": "^15.2.1",
"react-redux": "^4.4.5",
"redux": "^3.5.2",
"whatwg-fetch": "^1.0.0"
},
"devDependencies": {
"react-app-tools": "^1.1.1",
},
"scripts": {
"build": "react-app build",
"start": "react-app run",
}
}
```
### FAQ
- **Will I be able to customize Babel, Webpack, Browsersync and other configs?**
Yes, there will be an easy way to extend/tweak the default configs (Webpack, Babel, PostCSS etc.)
- **Will I be able to customize automation scripts?**
Yes, there will be an easy way to add or customize the existing scripts (`build`, `run` etc.)
### Preview

https://github.com/kriasoft/react-app

Start with webpack.config.js, package.json, and the existing build and run automation scripts. Compare them with the proposed react-app and react-app-tools packages, then determine how the configuration and scripts should be extracted and customized. Done means the starter uses the standalone packages while retaining the documented build, run, and testing workflow.

Written by the indexing model from the issue text.