include site logo if available

layouts/partials/header.html

@@ -1,2 +1,6 @@
+{{ with resources.Get site.Params.logo.src }}
+  <img src="{{ .RelPermalink }}" width="{{ .Width }}" height="{{ .Height }}" alt="{{ site.Params.logo.alt }}">
+{{ else }}
   <h1>{{ site.Title }}</h1>
+{{ end }}
 {{ partial "menu.html" (dict "menuID" "main" "page" .) }}