metadata: If set, use the summary parameter for `og:description`

Close #518

layouts/partials/header.html

@@ -15,6 +15,8 @@
   {{ if .IsPage }}
     {{ if .Params.abstract }}
       {{ $scr.Set "description" .Params.abstract }}
+    {{ else if .Params.summary }}
+      {{ $scr.Set "description" .Params.summary }}
     {{ else }}
       {{ $scr.Set "description" .Summary }}
     {{ end }}