Add .json as default extension to webpack config

Refer webpack/webpack#184 and webpack/webpack@53c16c8

Author: addnab

tools/webpack.config.js

@@ -68,7 +68,7 @@ const config = {
   ],
 
   resolve: {
-    extensions: ['', '.webpack.js', '.web.js', '.js', '.jsx'],
+    extensions: ['', '.webpack.js', '.web.js', '.js', '.jsx', '.json'],
   },
 
   module: {