Added Google search console tracking code

Search console provides more detail on search terms and sources visitors are using to find us.

_includes/head.html

@@ -2,7 +2,7 @@
   <meta charset="utf-8">
   <meta http-equiv="X-UA-Compatible" content="IE=edge">
   <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
-
+  <meta name="google-site-verification" content="Dz5XBAl8txqjo7NQmxz3lvWtAovEOlhABDqCewGdknU" />
   <title>{% if page.title %}{{ page.title | escape }}{% else %}{{ site.title | escape }}{% endif %}</title>
   <meta name="description" content="{% if page.excerpt %}{{ page.excerpt | strip_html | strip_newlines | truncate: 160 }}{% else %}{{ site.description }}{% endif %}">