support for custom message

[Shantanu Gadgil]

Hi,
Along with the current info shown, would it be possible to show a custom text message?
Something like (https://github.com/hashicorp/http-echo).

This would help to test/validate the load-balancing behaviors of LB like gobetween, Fabio, Traefik, HAProxy, etc.

Regards,
Shantanu