Add support for a "strict" mode

The current yaml loader is not strict: it ignores any unknown keys in the loaded file.

This, together with the fact that config keys are often automatically deduced, can be a bit error prone. Setting `foo-bar` in the config file, when you actually meant `foo-baz` may go unnoticed.

It would be therefore nice to have the possibility of a _strict_ loader, similar to yaml's strict decoding mode.