Add '24hour-delta' timestamp option

When enabled this option will timestamp new lines with the time elapsed
since the line before.

This is a very useful feature to identify which events takes the most
time.

src/options.h

@@ -31,6 +31,7 @@ enum timestamp_t
     TIMESTAMP_NONE,
     TIMESTAMP_24HOUR,
     TIMESTAMP_24HOUR_START,
+    TIMESTAMP_24HOUR_DELTA,
     TIMESTAMP_ISO8601,
     TIMESTAMP_END,
 };