Add meta tags for author, and don't allow old author param

layouts/partials/post-metadata.html

@@ -20,20 +20,10 @@
     {{ end }}
     {{ end }}
 
-    {{ $authorString := "" }}
-
     {{ with .Params.authors }}
-    {{ $authorString = delimit . ", " }}
-    {{ else }}
-    {{ with .Params.author }}
-    {{ $authorString = . }}
-    {{ end }}
-    {{ end }}
-
-    {{ with $authorString }}
     <span class="post-meta-item">
         {{ partial "icon.html" "user-circle" }}
-        {{ . }}
+        {{ delimit . ", " }}
     </span>
     {{ end }}