Make the Nominatim URL Configurable

[gelinger777]

This will make the URL of nominatim configurable. Please see my next pull request for the option change in /src/leaflet_options.js

[DennisOSRM]

• Implemented in PR feat: modifiable nominatim endpoint #385: commit c7b5379 — added leafletOptions.nominatim.path and passed it into geocoder; updated src/leaflet_options.js, src/geocoder.js, src/index.js and scripts/replace.js.
• Runtime support: commit 3a7ba48 — exposes NOMINATIM_ENDPOINT in docker/entrypoint.sh and reads it into config.json; leaflet_options now uses config.NOMINATIM_ENDPOINT if present.
• (Earlier related: 090e00c added initial modifiable-nominatim work.)

How to use: set NOMINATIM_ENDPOINT (build env or container env) — it will override the default Nominatim URL.