Add ES-modules build (#175)

Author: goto-bus-stop

.babelrc

@@ -1,6 +1,6 @@
 {
   "presets": [
-    "es2015",
+    "./build/preset-es2015",
     "stage-0"
   ]
 }

.gitignore

@@ -1,6 +1,7 @@
 node_modules
 coverage
 *.log
+es
 lib
 dist
 .idea

README.md

@@ -12,6 +12,8 @@
 npm install --save redux-actions
 ```
 
+The [npm](https://www.npmjs.com) package provides a [CommonJS](http://webpack.github.io/docs/commonjs.html) build for use in Node.js, and with bundlers like [Webpack](http://webpack.github.io/) and [Browserify](http://browserify.org/). It also includes an [ES modules](http://jsmodules.io/) build that works well with [Rollup](http://rollupjs.org/) and [Webpack2](https://webpack.js.org)'s tree-shaking.
+
 If you don’t use [npm](https://www.npmjs.com), you may grab the latest [UMD](https://unpkg.com/redux-actions@latest/dist) build from [unpkg](https://unpkg.com) (either a [development](https://unpkg.com/redux-actions@latest/dist/redux-actions.js) or a [production](https://unpkg.com/redux-actions@latest/dist/redux-actions.min.js) build). The UMD build exports a global called `window.ReduxActions` if you add it to your page via a `<script>` tag. We *don’t* recommend UMD builds for any serious application, as most of the libraries complementary to Redux are only available on [npm](https://www.npmjs.com/search?q=redux).
 
 ## Usage

build/preset-es2015.js

@@ -0,0 +1,7 @@
+const esOptions = process.env.BABEL_ENV === 'es' ? { modules: false } : {};
+
+module.exports = {
+  presets: [
+    ['es2015', esOptions]
+  ]
+};

package.json

@@ -3,20 +3,24 @@
   "version": "1.1.0",
   "description": "Flux Standard Action utlities for Redux",
   "main": "lib/index.js",
+  "module": "es/index.js",
+  "jsnext:main": "es/index.js",
   "scripts": {
+    "build:es": "cross-env BABEL_ENV=es babel src --out-dir es --ignore *-test.js",
     "build:commonjs": "babel src --out-dir lib --ignore *-test.js",
     "build:umd": "cross-env NODE_ENV=development webpack",
     "build:umd:min": "cross-env NODE_ENV=production webpack",
-    "build": "npm run clean && npm run build:commonjs && npm run build:umd && npm run build:umd:min",
-    "clean": "rimraf lib",
-    "lint": "esw src webpack.config --color",
+    "build": "npm run clean && npm run build:es && npm run build:commonjs && npm run build:umd && npm run build:umd:min",
+    "clean": "rimraf lib es",
+    "lint": "esw build src webpack.config --color",
     "lint:fix": "npm run lint -- --fix",
     "lint:watch": "npm run lint -- --watch",
     "prepublish": "npm run lint && npm run test && npm run build",
     "test": "mocha --compilers js:babel-register src/**/*-test.js",
     "test:watch": "npm run test -- --watch src/**/*-test.js"
   },
   "files": [
+    "es",
     "lib",
     "dist"
   ],