Change countries option after init

Nobody has claimed this yet.

I'm using the MapboxGeocoder with the `countries` option set to a default country, but I would like to update this option if they change the country they are wanting to search in.

Current code:

```javascript
// Add the control to the map.
const geocoder = new MapboxGeocoder({
accessToken: mapboxgl.accessToken,
mapboxgl: mapboxgl,
marker: false,
countries: 'nz'
});
```

I'm using the [chosen plugin from harvest](https://harvesthq.github.io/chosen/), so I have the `on change` function working, I need to know if it's possible to change the `countries` option when they change the country so that results searched for will only return for the current country they have selected

```javascript
// Country selection changed
$('#country_id').on('change', function(evt, params) {
// Check it's not empty
if (params) {
// code to go in here to change 'countries' option
}
});
```

Start at the MapboxGeocoder initialization and the `countries` option, then trace how the chosen change handler should interact with the geocoder instance. Confirm whether the documented API supports changing countries after initialization; done means searches are restricted to the selected country, with the empty-selection case handled.

Written by the indexing model from the issue text.