Await `window.PagedConfig.before` and / or call `after` if set

Currently pagedjs-cli does not await window.PagedConfig.before and does not call window.PagedConfig.after.

For example:

Including a script with --additional-script containing

window.PagedConfig = {
  before: () => {
      return new Promise(r => setTimeout(r, 1000));
  }
};

does nothing.