default placeholder text should be localized if language option provided

[vfoss]

The placeholder text in the search bar for the geocoder remains "Search" (in English), even when the language of the map is changed, as in
let geocoder = new MapboxGeocoder({accessToken: mapboxgl.accessToken, language: 'es' });. I cannot figure out a clean way to change it.

[andrewharvey]

@vfoss You can set it with the placeholder option.

But let's leave this ticket open to make the default placeholder set by the first language if provided.

[andrewharvey]

Done in https://github.com/mapbox/mapbox-gl-geocoder/tree/version4 by #201, slated for v4 release.