catch_workers_output php fastcgi compatibility does not exist

In HHVM 3.2 there is still no support for `catch_workers_output` that php-fpm allows.

``` bash
catch_workers_output = yes
; Redirect worker stdout and stderr into main error log.
; If not set, stdout and stderr will be redirected to /dev/null according to FastCGI specs.
```

This means there is no way to automatically capture the remote ip address in the error logs since there is no way to control the format of HHVM error logs. (With `catch_workers_output` STDERR would be sent to nginx or apache and those logs would show the ip that was sent to fastcgi that caused the error.)

If this is difficult to implement, alternatively please give us a way to control errorlog format.

Even if `%a` cannot work in a fastcgi environment, in theory `%{REMOTE_ADDR}e` should (if HHVM supports).

I tried to use `%a` and `%{REMOTE_ADDR}e` in the `--extra-headers` command line option for logs but unfortunately `extra-headers` seems to be printed literally without trying to interpret format variables.