Allow specifying extra layer options in layers.yml

[AntonKhorev]

Now you can (re)define other options accepted by Leaflet tile layers, for example url or maxZoom. Also you define new tile layers that are not present in leaflet.osm.js because leafletOsmId is no longer mandatory.

This lets you add, for example, dark version of Transport Map by adding to config/layers.yml:

- url: 'https://{s}.tile.thunderforest.com/transport-dark/{z}/{x}/{y}{r}.png?apikey={apikey}'
  maxZoom: 21
  code: "U"
  keyId: "transportmapdark"
  nameId: "transport_map"
  apiKeyId: "THUNDERFOREST_KEY"
  credit:
    id: "thunderforest_credit"
    children:
      thunderforest_link:
        id: "andy_allan"
        href: "https://www.thunderforest.com/"

[tomhughes]

Nice - making it easier for people reusing the software for other sites to change the map layers has been a long standing problem.