Support strict validation for the configuration file

[stsewd]

We can support strict validation in the configuration file (v2 only at this point).

What do I mean by strict validation?

Just to validate that there aren't other keys in the config file

Why do we want this?

It can prevent typos :). It should be straightforward to implement this in the current validation steps. Just pop each value whenever it is used.

[davidfischer]

I'm +1 for this as it will prevent future bugs.

[humitos]

I think this is something good to have. I will avoid confusions to users, and by given them a good validation error message, it's something that they can fix by themselves.