Feature request: Add `nilable` or `maybe` function

[Munksgaard]

This is a minor thing, but I find myself writing StreamData.one_of([nil, some_other_generator]) quite a lot. Would there be interest in a PR that adds a StreamData.maybe/1 generator that makes another generator nilable?

Of course, there's a discussion about what the right distribution of values should be here. The simple one_of from above makes it so that nil is returned 50% of the time, which may or may not be the desired outcome. Feedback welcome!

[whatyouhide]

I'm ok with nullable(generator, opts \\ []) with options supporting ratio: float() in 0.0 → 1.0. Wanna get a PR up? 🙃

[Munksgaard]

Of course! I've opened #221.