SEO primary image (fixes #538)

Author: cmbuckley

_includes/seo.html

@@ -56,6 +56,7 @@
     <script type="application/ld+json">{
       "@context": "https://schema.org",
       "@type": "ProfilePage",
+      "primaryImageOfPage": "{{ site.url }}/assets/img/headshot.jpg",
       "mainEntity": {
         "@type": "Person",
         "name": "Chris Buckley",