[352] Extract the manifest file to make vendor.js be cached longer

As described in this documentation: https://webpack.js.org/guides/code-splitting-libraries/. In the current version of nuxt, if I add this config to webpack plugins, there is an error claiming "Multiple assets emit to the same filename nuxt.bundle.js":

new webpack.optimize.CommonsChunkPlugin({
      names: ['vendor', 'manifest']
})