Add configurable geocoding (Search) URL

[ampledata]

As an administrator, I would like the ability to customize the geocoding URL to use my self-hosted API.

Currently the geociding Search feature is hard-coded to use OSM's Nominatim (see worldmap.js:629/doSearch())

Add a config parameter to customize this URL.

I don't yet know the code-base well enough to include a PR in this Issue, but would appreciate feedback on whether this should be a Node configuration parameter, or a Worldmap command that can get passed in the payload.

[dceejay]

@ampledata How about if I allow setting it via an (external) environment variable - eg WORLDMAP_GEOCODING_URL ?

but... errr - unless your geocoder returns things in exactly the same format (IE a json array of objects with a property called boundingBox - I won't know what to do with the response...