CRA_BABEL_PRESET_FILE to handle optional dependencies.

Created by: jsdevel

• This will allow for optional babel dependencies to be added without ejecting cra.
• CRA_BABEL_PRESET_FILE=relative/path/to/babel/preset can be added to package.json.

I verified this locally by copying index.js to node_modules/babel-preset-react-app/index.js within one of my projects. Here is the package.json for my project:

{
  "name": "my-app",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "babel-plugin-relay": "^1.1.0",
    "react": "^15.6.1",
    "react-dom": "^15.6.1",
    "react-router-dom": "^4.1.1",
    "react-scripts": "1.0.10",
    "shared-git-hooks": "^1.2.1"
  },
  "scripts": {
    "start": "CRA_BABEL_PRESET_FILE=.babel-preset-react-app react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test --env=jsdom",
    "eject": "react-scripts eject"
  },
  "homepage": "."
}

I'm doing this because I want to use relay modern in my app. My .babel-preset-react-app.js file is exactly what @maletor had in PR https://github.com/facebookincubator/create-react-app/pull/2343

cc @gaearon Is this an acceptable approach per your comment https://github.com/facebookincubator/create-react-app/pull/2343#issuecomment-311409607 ?

Also relates to #2001 (closed)