report runtime of script in generated sitemap

templates/sitemap.html.j2

@@ -4,7 +4,7 @@
 </head>
 <body>
 <p>
-Crawled {{ urlcount }} URLs on {{ base_url }}
+Crawled {{ urlcount }} URLs on {{ base_url }} in ~{{ runtime }} seconds.
 <ul>
 {% for url in urls %}
   <li><a href="{{ url }}">{{ url }}</a></li>