/register endpoint, permissions

Hi Maurits,

Once again, great job on a welcomed feature (/register endpoint and dbAuth.registerUser middleware config)!

Do you think if it would be feasible to add a config (or use the same dbAuth.registerUser config) that would allow us to specify what operations should be allowed on this endpoint (e.g.: allow-overwrite / unique, something like that) - maybe, failing if we try something that is not allowed, etc. ?

Scenario: I have a predefined user (let's call it "super-admin"). This super-admin user, after being logged in, will try to register a new user via the /register endpoint.
Now, the api allows us to repeat this operation, basically adding the same user to the db multiple times. Could we stop this behaviour via a config option? This is just for discussion, and, maybe, future enhancement of this (really great) endpoint.

Cheers! Keep the good work comin'!