create gtag plugin, same as analytics, but updated

plugins/gtag.lisp

@@ -0,0 +1,21 @@
+(defpackage :coleslaw-gtag
+  (:use :cl)
+  (:export #:enable)
+  (:import-from :coleslaw #:add-injection))
+
+(in-package :coleslaw-gtag)
+
+(defvar *analytics-js*
+"<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src='https://www.googletagmanager.com/gtag/js?id=~a'></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', '~a');
+</script>")
+
+(defun enable (&key tracking-code)
+  (let ((snippet (format nil *analytics-js* tracking-code tracking-code)))
+    (add-injection (constantly snippet) :head)))