Generate html-manifest.json of html assets.

Created by: patope

It's non-trivial to reconstruct html page using asset-manifest.json currently. See: https://github.com/facebook/create-react-app/issues/5513

This change adds alternative asset file for that purpose. ExtractHtmlManifestPlugin extracts html page assets as defined here https://github.com/jantimon/html-webpack-plugin/blob/master/lib/hooks.js#L21 to separate file html-manifest.json. This file can be used to reconstruct html page.

example of generate html-manifest.json:

{
  "publicPath": "/",
  "js": [
    "/static/js/runtime~main.229c360f.js",
    "/static/js/1.82dafdb5.chunk.js",
    "/static/js/main.c9b66095.chunk.js"
  ],
  "css": [
    "/static/css/main.90a4e9dc.chunk.css"
  ]
}