Add error_log_format config option, similar to access_log_format

[chrisinmtown]

Please consider adding a new configuration option error_log_format that works much like the existing config option access_log_format, except of course for the error log (option errorlog) output format. I'm not the only one who would like this, please see these SO questions without answers:

https://stackoverflow.com/questions/45088749/how-can-i-configure-gunicorn-to-use-a-consistent-error-log-format
https://stackoverflow.com/questions/61763378/how-to-format-gunicorn-error-logs-using-config-file

I searched the open issues without finding anything close, hope I didn't miss anything obvious.

Thanks for listening!