Add a segmenter that is non-deterministic

In contrast with the fixed percentage segmenter which is deterministic based on the subject ID.

It's mostly intended for testing purposes: by using randomization, people need to do proper stubbing in  tests to get deterministic test results. This may catch some bugs related to experiments, e.g. the bahevior of one of the groups not bing tested at all.

@jeromecornet @peterjm @pseudomuto