Added timestamp format "epoch-usec"

This timestamp format will print the seconds since epoch along with
subdivision in microseconds.

Example:
 [1748009585.087083] tio v3.9-8-g2fb788f-dirty
 [1748009585.087156] Press ctrl-t q to quit
 [1748009585.087683] Connected to /dev/ttyUSB0

src/timestamp.h

@@ -29,6 +29,7 @@ typedef enum
     TIMESTAMP_24HOUR_DELTA,
     TIMESTAMP_ISO8601,
     TIMESTAMP_EPOCH,
+    TIMESTAMP_EPOCH_USEC,
     TIMESTAMP_END,
 } timestamp_t;