Add a cookie or some other mechanism to show debug bar on front-end when not logged

[d-packs]

A tool to investigate rewrite rules for visitors would be most welcome.
Ideally it would be best for this feature to not interfere with normal functioning of the site for regular visitors, but the developer should be able to trigger it via some mechanism (maybe based on IP?).

It would be great if this option would be added via some config constant, or any other means.