Add kea example (#3262)

examples/with-kea/.babelrc

+{
+  "presets": [
+    "next/babel"
+  ],
+  "plugins": [
+    "transform-decorators-legacy"
+  ]
+}

examples/with-kea/README.md

+[![Deploy to now](https://deploy.now.sh/static/button.svg)](https://deploy.now.sh/?repo=https://github.com/zeit/next.js/tree/master/examples/with-kea)
+
+# kea example
+
+## How to use
+
+Download the example [or clone the repo](https://github.com/zeit/next.js):
+
+```bash
+curl https://codeload.github.com/zeit/next.js/tar.gz/master | tar -xz --strip=2 next.js-master/examples/with-kea
+cd with-kea
+```
+
+Install it and run:
+
+```bash
+npm install
+npm run dev
+```
+
+Deploy it to the cloud with [now](https://zeit.co/now) ([download](https://zeit.co/download))
+
+```bash
+now
+```

examples/with-kea/package.json

+{
+  "name": "with-kea",
+  "version": "1.0.0",
+  "scripts": {
+    "dev": "next",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "kea": "^0.27.3",
+    "next": "^4.1.4",
+    "next-redux-wrapper": "^1.3.4",
+    "prop-types": "^15.6.0",
+    "react": "^16.1.0",
+    "react-dom": "^16.1.0",
+    "react-redux": "^5.0.6",
+    "redux": "^3.7.2",
+    "reselect": "^3.0.1"
+  },
+  "license": "ISC",
+  "devDependencies": {
+    "babel-plugin-transform-decorators-legacy": "^1.3.4"
+  }
+}

examples/with-kea/pages/index.js

+import React from 'react'
+import { initStore } from '../store'
+import withRedux from 'next-redux-wrapper'
+import PropTypes from 'prop-types'
+import { kea } from 'kea'
+
+@kea({
+  path: () => ['kea'],
+  actions: () => ({
+    increment: amount => ({ amount }),
+    decrement: amount => ({ amount })
+  }),
+  reducers: ({ actions }) => ({
+    counter: [
+      0,
+      PropTypes.number,
+      {
+        [actions.increment]: (state, payload) => state + payload.amount,
+        [actions.decrement]: (state, payload) => state - payload.amount
+      }
+    ]
+  }),
+  selectors: ({ selectors }) => ({
+    doubleCounter: [
+      () => [selectors.counter],
+      counter => counter * 2,
+      PropTypes.number
+    ]
+  })
+})
+class App extends React.Component {
+  render () {
+    return (
+      <div>
+        <p>Double Counter: {this.props.doubleCounter}</p>
+        <button type='button' onClick={() => this.actions.increment(1)}>Increment</button>
+        <button type='button' onClick={() => this.actions.decrement(1)}>Decrement</button>
+      </div>
+    )
+  }
+}
+
+export default withRedux(initStore)(App)

examples/with-kea/store.js

+import { keaReducer } from 'kea'
+import { createStore, compose, combineReducers } from 'redux'
+
+const reducers = combineReducers({
+  kea: keaReducer('kea')
+})
+
+const reduxDevTools =
+  typeof window !== 'undefined' && window.__REDUX_DEVTOOLS_EXTENSION__
+    ? window.__REDUX_DEVTOOLS_EXTENSION__()
+    : f => f
+
+export const initStore = () => {
+  return createStore(
+    reducers,
+    compose(reduxDevTools)
+  )
+}