Allow themes to build web bundles

In order to comply with modern fast loading webpages it should be possible for Hexo themes to use tools like https://github.com/webpack so we can serve optimized bundles of javascript and css to the browser.

I'm really looking forward to either a piece of documentation on how to achieve that or something like a hook which we can attach to the generate process.

E.g. check the [lighthouse](https://chrome.google.com/webstore/detail/lighthouse/blipmdconlkpinefehnmjammfjpmpbjk?hl=nl) scoring of [my blog](https://marcofranssen.nl).