I should be able to exclude someone from an experiment at any time

[AndyStabler]

At some point I can set up an experiment like so

let andyAb = new AndyAB("an experiment")
.withCohorts("control", "treatment")
.enrol()
.whenIn("control", () => { ... })
.whenIn("treatment", () => { ... });

andyAb.getCohort(); // treatment

I should also be able to explicitly exclude users at some point later on like so:

object.onClick = function() {
  let andyAb = new AndyAB("an experiment");
  andyAb.exclude("exclusion cohort");
  andyAb.getCohort(); // exclusion cohort
};