Create asset filenames mapping on the build output

Created by: lukyth

To resolve #600 (closed)

I use webpack-manifest-plugin on Webpack production configuration to create a file named asset-manifest.json which contain a mapping of all asset filenames to their corresponding output file. asset-manifest.json will be located at the root of output folder.

Here's the example from a newly created project.

{
  "main.css": "static/css/main.9a0fe4f1.css",
  "main.css.map": "static/css/main.9a0fe4f1.css.map",
  "main.js": "static/js/main.c9af8c27.js",
  "main.js.map": "static/js/main.c9af8c27.js.map",
  "static/media/logo.svg": "static/media/logo.5d5d9eef.svg"
}

You can see that it doesn't map those files from public/. What do you think?