added set_gtime_format for generation formatted timestamp

simple generation of formatted string
useful for time-separation
for example as hourly part of statistic directly in redis using redis2-nginx-module where part of redis keyname generated by set_gtime_format

```
        location /today.html {
            default_type plain/html;
            set_local_today $today;
            set_gtime_format $today2 "%Y-%m-%d %H:%M:%S";
            set_gtime_format $today3 "%Y%m%d%H%M%S";
            set_gtime_format $today4 "%x";
            set_gtime_format $today5 "";
            echo $today;
            echo $today2;
            echo $today3;
            echo $today4;
            echo $today5;
            break;
        }
```