Introduce Webpack (#24)

This does not represent any functional change to the behavior of the library. Since this component is getting reasonably complex, this diff adds bundling of files via Webpack. I have broken off a first small piece into geometry.js just as an example, and we should continue to split things up further.

Notes:
- Our webpack config emits a umd compatible module. Currently in master we only support es6 style modules.
- See the notes in ./copy-to-example.sh to see how we are using the dist of this lib in the example app
- Some useful resources: https://webpack.js.org/guides/author-libraries/ and https://github.com/kalcifer/webpack-library-example

.babelrc

@@ -1,8 +1,8 @@
 {
-  "only": "./index.jsx",
-  "plugins": ["transform-react-jsx"],
+  "only": ["src/**"],
   "presets": [
-    ["env", {
+    "@babel/preset-react",
+    ["@babel/preset-env", {
       "targets": {
         "browsers": ["ie >= 9"]
       }

README.md

@@ -127,3 +127,6 @@ MapInteraction doesn't require any props. It will control its own internal state
   ...MapInteractionCSS.propTypes,
 }
 ```
+
+## Development
+Please feel free to file issues or put up a PR. There are currently no automated tests, but there is an example application in the `example` directory. When you build this library it will inject the dist into the `example/node_modules` so it can be imported by that application. Annoyingly, create-react-app doesn't watch the `node_modules` directory so you need to manually restart the example app with `npm start`.

copy-to-example.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+# This is a hack to work around limitations in create-react-app
+# E.g.: https://github.com/facebook/create-react-app/issues/3883
+# We completely copy over the dist of this library into the ./example folder's node_modules
+
+mkdir -p example/node_modules/react-map-interaction/dist
+cp dist/react-map-interaction.js example/node_modules/react-map-interaction/dist
+cp package.json example/node_modules/react-map-interaction/

example/.env

@@ -0,0 +1 @@
+SKIP_PREFLIGHT_CHECK=true

example/src/App.js

@@ -1,6 +1,6 @@
 import React, { Component } from 'react';
 
-// assumes that you have run `npm link react-map-interaction`
+// See ../copy-to-example.sh
 import { MapInteractionCSS } from 'react-map-interaction';
 
 class App extends Component {