Use cosmic config to allow config to be in an external file

[siddharthkp]

You can now keep the config in a separate file if you want - bundlesize.config.js

Format:

{
  "files": [
    {
      "path": "./dist.js",
      "maxSize": "3 kB"
    }
  ]
}

If you do not want to crowd your root directory, you can mention a different path:

bundlesize --config configs/bundlesize.json

Check rendered README for the updated docs.

[Haroenv]

nice new changes in the readme!