layouts: Add imageConfig to set height and width

Hugo 0.18 introduced the imageConfig function which allows you to
access the height and width of images, which makes bots consuming
the structured data happy.

layouts/_default/single.html

@@ -10,7 +10,11 @@
   {{- with .Params.images -}}{{ range first 1 . }}
   "image": {
     "@type": "ImageObject",
-    "url": "{{ . | absURL }}"
+    "url": "{{ . | absURL }}",
+    {{ with (imageConfig (printf "/static/%s" .)) }}
+    "height": "{{ .Height }}",
+    "width": "{{ .Width }}"
+    {{ end }}
   },
   {{- end -}}{{ end }}
   "url": "{{ printf "%s" .Permalink }}",