Added a auto option to trace to show the slowest stack trace.

Usage:

```bash
$ grind-pilot trace /tmp/cachegrind.out.10304 auto
```

```
Inclusive Self      Callstack
5001.0690    0.0360 {main}
5001.0330    0.0960   main
2000.4120    0.0420     func1
1000.1930    0.0990       func2
1000.0940 1000.0940         php::sleep
--------------------------------------------------
 No children found.
```