Added an option to the JSON formatter to use ISO 8601 for timestamps.

- It's not *exactly* ISO 8601 which doesn't seem to support
   subseconds, but subseconds are very important to us and
   most things that support ISO8601 seem to also support subseconds
   in the way I'm implemented it.

scripts/base/frameworks/logging/writers/ascii.bro

@@ -24,6 +24,12 @@ export {
 	## This is also available as a per-filter $config option.
 	const use_json = F &redef;
 
+	## By default, the JSON formatter will use double values for timestamps
+	## which represent the number of seconds from the UNIX epoch.  By setting
+	## this to 'T', it will use the 8601 format.  This is also available as 
+	## a per-filter $config option.
+	const json_iso_timestamps = F &redef;
+
 	## If true, include lines with log meta information such as column names
 	## with types, the values of ASCII logging options that are in use, and
 	## the time when the file was opened and closed (the latter at the end).