Also adds to manifest all assets which are not in a chunk

[Kronuz]

Some times, when loaders or plugins add assets to the bundle (file-loader for example), assets end up without existing in any particular chunk, still they're generated assets. This pull request adds an "empty" chunk in which such assets get added.

Example of a manifest which has assets that are not in any chunk (file-loader adding a png):

{
  "main": {
    "js": "main.js",
    "css": "main.css"
  },
  "": {
    "png": "image.png"
  }
}

[d4rky-pl]

bump