add experimental babel-plugin-macros support (facebook#3675)

* add experimental babel-plugin-macros support

closes facebook#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](facebook#2730 (comment))
and it worked well.

* Pin the version

packages/babel-preset-react-app/index.js

@@ -7,6 +7,9 @@
 'use strict';
 
 const plugins = [
+  // Experimental macros support. Will be documented after it's had some time
+  // in the wild.
+  require.resolve('babel-plugin-macros'),
   // Necessary to include regardless of the environment because
   // in practice some other transforms (such as object-rest-spread)
   // don't work without it: https://github.com/babel/babel/issues/7215

packages/babel-preset-react-app/package.json

@@ -12,6 +12,7 @@
   ],
   "dependencies": {
     "babel-plugin-dynamic-import-node": "1.1.0",
+    "babel-plugin-macros": "2.0.0",
     "babel-plugin-syntax-dynamic-import": "6.18.0",
     "babel-plugin-transform-class-properties": "6.24.1",
     "babel-plugin-transform-es2015-destructuring": "6.23.0",