Enable source maps for webpack dev mode

Following along with facebook/create-react-app#924,
enable source maps by default.  This lets us see the actual code in chrome
and firefox dev tools.  Without the source maps, you view the transpiled
code in dev tools.

config/webpack.config.dev.js

@@ -13,11 +13,11 @@ var webpackConfigCommons = require('./webpack.config.commons')
 // It is focused on developer experience and fast rebuilds.
 // The production configuration is different and lives in a separate file.
 module.exports = {
-  // This makes the bundle appear split into separate modules in the devtools.
-  // We don't use source maps here because they can be confusing:
-  // https://github.com/facebookincubator/create-react-app/issues/343#issuecomment-237241875
-  // You may want 'cheap-module-source-map' instead if you prefer source maps.
-  devtool: 'eval',
+  // Integrate https://github.com/facebook/create-react-app/pull/924 here.
+  // You may want 'eval' instead if you prefer to see the compiled output in DevTools.
+  // See the discussion in https://github.com/facebookincubator/create-react-app/issues/343.
+  devtool: 'cheap-module-source-map',
+
   // These are the "entry points" to our application.
   // This means they will be the "root" imports that are included in JS bundle.
   // The first two entry points enable "hot" CSS and auto-refreshes for JS.