timestamp string format for SQLite;

--- a/src/Pure/General/date.scala	Thu Feb 09 22:40:15 2017 +0100
+++ b/src/Pure/General/date.scala	Thu Feb 09 23:00:56 2017 +0100
@@ -39,6 +39,7 @@
     val default: Format = Format("dd-MMM-uuuu HH:mm:ss xx")
     val date: Format = Format("dd-MMM-uuuu")
     val time: Format = Format("HH:mm:ss")
+    val timestamp: Format = Format("uuuu-MM-dd HH:mm:ss.SSS x")
   }
 
   abstract class Format private