Add custom directory structure and filename for offline tilesets

Supporting offline tilesets in XYZ URL(path/to/tileset/{z}/{x}/{y}.png) format or a custom with a template is requested. A template placeholder for the parameter for `osm.mapping.offline.directory` could be added.

For reference, check the openlayers' XYZ source example: https://openlayers.org/en/latest/examples/xyz.html