Add comment counter to page metadata

Also move the Isso script tag from comments.html to the <head>, since it
essentially needs to load on all pages.

Fixes #94

layouts/_partials/page-metadata/post-full.html

@@ -21,6 +21,7 @@
         {{ partial "page-metadata/item/readingtime.html" . }}
         {{ partial "page-metadata/item/edithistory.html" . }}
         {{ partial "page-metadata/item/viewraw.html" . }}
+        {{ partial "page-metadata/item/comments-counter.html" . }}
 
         {{ partial "page-metadata/item/categories.html" . }}