Add Ketch smart tag (#173)

Author: eric-schneider

src/partials/head-prelude.hbs

@@ -9,3 +9,6 @@
     })(window,document,'script','dataLayer','{{this}}');</script>
     <!-- End Google Tag Manager -->
     {{/with}}
+    {{#with site.keys.ketchSmartTagUrl}}
+    <script>!function(){window.semaphore=window.semaphore||[],window.ketch=function(){window.semaphore.push(arguments)};var e=document.createElement("script");e.type="text/javascript",e.src="{{this}}",e.defer=e.async=!0,document.getElementsByTagName("head")[0].appendChild(e)}();</script>
+    {{/with}}