Add a "sass" option to the UI question when generating a new app.

[NullVoxPopuli]

Currently, sass is included by default.

Some projects may opt to use some variant of CSS-in-JS / inline-styles over sass, so when sass is not selected during project creation, we need to:

• not generate components with sass files
• not include a global sass file.

This means that the generated app needs to know about what options it was generated with.
I propose a .react-cli.js file that keeps track of the options that were used to create it.

an example of this file may be something like:

'use strict';

module.exports = {
  appBlueprintVersion: '0.1.1',

  features: {
    redux: true,
  },
  styles: {
    sass: false,
    materialUi: true,
  }
}

then when generating a component, we can look at this file, see if the app has sass or material or whatever, and adjust the output accordingly.