Support watchOptions configuration. (#287)

This commit causes the watchOptions in the configuration provided
to `react-storybook` to be passed on to `webpack-dev-middleware`.

This allows those who need these options for hot reloading to be able
to use them again.

src/server/middleware.js

@@ -23,6 +23,7 @@ export default function (configDir) {
   const devMiddlewareOptions = {
     noInfo: true,
     publicPath: config.output.publicPath,
+    watchOptions: config.watchOptions || {},
   };
 
   const router = new Router();