Per-experiment sampling?

When using the `science` method in the `Scientist` module, I want to supply an extra parameter that would be used inside the `enabled?` method to determine whether to run the experiment.

The intention is to be able to control, on a per-experiment basis, the percentage of time that the experiment will actually run.

Is something like this possible?

Start by reading the `science` method and the `enabled?` method in the Scientist module to understand how an experiment is selected for execution. Define the per-experiment sampling behavior and its API before implementation; done should include coverage showing that the configured percentage controls whether the experiment runs.

Written by the indexing model from the issue text.