Improve logging functionality

The format of timestamps is now configurable via a new
--timestamp-format option. The default format is changed
to Unix epoch time with microsecond granularity.

Co-authored-by: Davide Pesavento <davidepesa@gmail.com>
Change-Id: I7dd7fb770becca7aa339e003b1111366f34ce925
diff --git a/README.md b/README.md
index 3b77b9b..ef34ee5 100644
--- a/README.md
+++ b/README.md
@@ -29,30 +29,40 @@
 ### `ndn-traffic-server`
 
     Usage: ndn-traffic-server [options] <Traffic_Configuration_File>
+
     Respond to Interests as per provided Traffic_Configuration_File.
     Multiple prefixes can be configured for handling.
     Set the environment variable NDN_TRAFFIC_LOGFOLDER to redirect output to a log file.
+
     Options:
-      -h [ --help ]           print this help message and exit
-      -c [ --count ] arg      maximum number of Interests to respond to
-      -d [ --delay ] arg (=0) wait this amount of milliseconds before responding to each Interest
-      -q [ --quiet ]          turn off logging of Interest reception/Data generation
+      -h [ --help ]                 print this help message and exit
+      -c [ --count ] arg            maximum number of Interests to respond to
+      -d [ --delay ] arg (=0)       wait this amount of milliseconds before responding to each Interest
+      -t [ --timestamp-format ] arg format string for timestamp output (see below)
+      -q [ --quiet ]                turn off logging of Interest reception and Data generation
 
 ### `ndn-traffic-client`
 
     Usage: ndn-traffic-client [options] <Traffic_Configuration_File>
+
     Generate Interest traffic as per provided Traffic_Configuration_File.
     Interests are continuously generated unless a total number is specified.
     Set the environment variable NDN_TRAFFIC_LOGFOLDER to redirect output to a log file.
+
     Options:
       -h [ --help ]                 print this help message and exit
       -c [ --count ] arg            total number of Interests to be generated
       -i [ --interval ] arg (=1000) Interest generation interval in milliseconds
-      -q [ --quiet ]                turn off logging of Interest generation/Data reception
+      -t [ --timestamp-format ] arg format string for timestamp output (see below)
+      -q [ --quiet ]                turn off logging of Interest generation and Data reception
+      -v [ --verbose ]              log additional per-packet information
 
 * These tools need not be used together and can be used individually as well.
 * Please refer to the sample configuration files provided for details on how to create your own.
 * Use the command line options shown above to adjust traffic configuration.
+* By default, timestamps are logged in Unix epoch format with microsecond granularity.
+  For custom output, the `--timestamp-format` option expects a format string using the syntax given in the
+  [Boost.Date_Time documentation](https://www.boost.org/doc/libs/1_71_0/doc/html/date_time/date_time_io.html#date_time.format_flags).
 
 ## Example