Add `storeConfigInMeta` option.

Allows opting out of using `<meta>` tag in `head` section for
configuration.

* If set to `true` the calculated config will be stored in a `<meta>`
  tag in `index.html`.
* If set to `false` the calculated config will be stored in
  the final `my-app-name.js` file.

Defaults to `true`.