add current_time option to out_stdout

Current `out_stdout` outputs messages as

```
2010-05-04 12:02:01 +0900 test: {"test":"test"}
```

where the time expresses the time parameter of messages, i.e., the time when the message is created or, the time written in application logs. 

`current_time` option enables to output the current time of *logging* which looks like

```
2014-04-24 18:49:23 +0900 2010-05-04 12:02:01 +0900 test: {"test":"test"}
```

so that we can tell the latency between receiving (logging) and sending (creating) of messages. 

PS. I am still wondering the log format and the option name.