upgrade webpack to ^2.6.0

This brings manager and docs up to the latest version of webpack 2.6
release. It will allow for future improvements that are only available
in webpack 2+.

Changes include:

- Updated syntax for webpack loaders. [link](https://webpack.js.org/guides/migrating/#module-loaders-is-now-module-rules)
- Use the babel-plugin-istanbul to instrument code for tests,
  as per recommendation by the webpack-contrib team. [link](webpack-contrib/istanbul-instrumenter-loader#44 (comment))
- Use `cross-env` for all npm scripts to improve compatibility.

.babelrc

@@ -14,6 +14,11 @@
   "env": {
     "development": {
       "presets": ["react-hmre"]
+    },
+    "test": {
+      "plugins": [
+        ["istanbul"]
+      ]
     }
   }
 }