Enhancement : add the possibility to force specific countries on search

[nelero]

When you search using the box : postcode, city, state and if you type a postcode which is in more than one country, the map is zooming out on Europe level or word level.
For many stores, they are specific to one or more countries.
It could be great to have an option in backend to precise that we want the search focusing on some countries.
In OSM for example, we can already add the parameter countrycodes to specify the countries searched in.

thanks