Support .json extension

Some libraries rely on it, and they work fine with browserify.
Let’s just do the right thing here by default.

config/webpack.config.dev.js

@@ -29,7 +29,7 @@ module.exports = {
     publicPath: '/'
   },
   resolve: {
-    extensions: ['', '.js'],
+    extensions: ['', '.js', '.json'],
   },
   resolveLoader: {
     root: paths.ownNodeModules,

config/webpack.config.prod.js

@@ -36,7 +36,7 @@ module.exports = {
     publicPath: publicPath
   },
   resolve: {
-    extensions: ['', '.js'],
+    extensions: ['', '.js', '.json'],
   },
   resolveLoader: {
     root: paths.ownNodeModules,