Implement organization membership endpoint

[domenkozar]

[domenkozar]

cc @andreasabel using this in production and it works well :)

[andreasabel]

@domenkozar Thanks for the PR! and sorry for the slow response.

Would you mind adding a test or example demonstrating your new feature? See

• examples: https://github.com/haskell-github/github/tree/master/samples
• tests: https://github.com/haskell-github/github/tree/master/spec/GitHub

I'd like minimum coverage of the features to get some evidence that they work and when they break because of upstream changes.

I am rebasing this onto master to fix the CI failure.

@Mergifyio rebase