Adds configuration for strict registration

Part of: #18 (closed)

This introduces a new configuration option that raises an exception if there isn't an experiment class registered for the experiment that's being run.

Meaning that if the new strict_registration configuration variable was true, and someone attempted to call experiment(:example) and no ExampleExperiment class can be auto loaded or resolved, an exception will be raised.

This is something I want to explore on gitlab, so we can see how many unregistered experiments we have. I feel like we can do a lot better reporting if we required experiments to be defined in a class, and in that way improve on our clarity for experiments.