feat: add filtersearch option to prefilter an OSM search

[trafficonese]

Based on Nominatims free-form-query:

In this form, the query can be unstructured. Free-form queries are processed first left-to-right and then right-to-left if that fails. So you may search for pilkington avenue, birmingham as well as for birmingham, pilkington avenue. Commas are optional, but improve performance by reducing the complexity of the search.

This is very useful if you want to enable OSM search only for some regions. It would fix #231