title: Force flutter_google_places to only display cities when using GooglePlacesAutocompleteWidget
body: <p>I'm using the flutter_google_places widget to search for cities.</p>
  
  <p>I've tried the AutoComplete example given with the widget and it works perfectly. But when I type "Paris" for example, I get many results of places in Paris (Paris, Paris Airport, Paris Expo, etc.). </p>
  
  <p>I only need cities within my App. So for example, when typing "Paris", I would only want "Paris (France), Paris (Texas), Paris (Tennessee), etc.</p>
  
  <p>I haven't found how to apply the "cities" filter. </p>
  
  <p>Here is the equivalent in Javascript : </p>
  
  <pre><code>var input = document.getElementById('searchField');
  var options = {
      types: ['(cities)']
  };
  
  autocomplete = new google.maps.places.Autocomplete(input, options);
  </code></pre>
  
  <p>Any idea?</p>