layouts: layer: Allow retrieving API key from config

The config key name is the layer name with '.' replaced with '_'. For example:

params:
  modules:
    leaflet:
      com_thunderforest_outdoors:
        apiKey: abcdef1234567890abcdef1234567890

If the key is specified in both the config file and the tag attribute, the
latter takes precedence.

Implements #6

layouts/shortcodes/leaflet-layer.html

@@ -9,10 +9,12 @@
 {{ $uniqueMapId := (printf "%s:%d" .Page.File.UniqueID .Parent.Ordinal) | md5 | safeJS }}
 
 {{ $id := .Get "id" }}
+{{ $configId := replace (.Get "id") "." "_" }}
+{{ $apiKey := default (index .Site.Params.modules.leaflet $configId "apiKey") (.Get "apiKey") }}
 <script>
 	map_options_{{ $uniqueMapId }}.layers.enabled.push({{ $id }});
 	map_options_{{ $uniqueMapId }}.layers["{{ $id }}"] = {
-		{{ with .Get "apiKey" }}
+		{{ with $apiKey }}
 		apiKey: '{{ . }}',
 		{{ end }}
 	};