added google_tag_manager_id from defaults.ini

conf/sample.ini

@@ -145,6 +145,9 @@ log_queries =
 # Google Analytics universal tracking code, only enabled if you specify an id here
 ;google_analytics_ua_id =
 
+# Google Tag Manager ID, only enabled if you specify an id here
+;google_tag_manager_id =
+
 #################################### Security ####################################
 [security]
 # default admin user, created on startup