* add experimental babel-plugin-macros support

closes #2730

This will remain undocumented until the brave have tried it in the wild.

**Test Plan:**

There's currently no established way to test changes to
`babel-preset-react-app`. But I did create
[`unmaintained-react-scripts-babel-macros`](https://www.npmjs.com/package/unmaintained-react-scripts-babel-macros)
[a while back](https://github.com/facebookincubator/create-react-app/issues/2730#issuecomment-328153982)
and it worked well.

* Pin the version