Expose build manifest

Created by: gaearon

We should include an additional file as the build output. This file should contain a mapping of asset file names so that tools can pick it up without having to parse index.html. This is useful for integrations like this: http://myappincome.co.uk/how-to-make-a-react-js-wordpress-plugin/.

AFAIK Webpack already provides access to such object via its API, we just need to figure out how to write it to a file, and decide on the filename.