Add pipe logger.

It's good for people coming from Apache.

```
CustomLog "|/usr/local/apache2/bin/rotatelogs /var/log/httpd.log.%Y%m%d 86400 0"
```

becomes

```
req-logger=pipe:/usr/local/apache2/bin/rotatelogs /var/log/httpd.log.%Y%m%d 86400 0
```