Document context support in before & after hooks

[novemberborn]

Fixes #1579.

Heads up @kugtong33.

[kugtong33]

👍 , I was heading on creating a PR for this, should I reference it still? :D , or nah

https://github.com/kugtong33/ava/blob/before-hook-context-document/readme.md

[novemberborn]

@kugtong33 looks like I beat you to it, sorry. Wanted to get this in so I can do a new release. Let me know if there's anything you'd like to see changed in my version though.

[kugtong33]

@novemberborn , I think the important part is to determine what values are shared/referenced and what values are independent in each test, and better examples would be fine, as I have added on my side we can give examples for referenced values and primitive values and how they can use contexts correctly.

We can also create a recipe for it, migrating declared variables outside of hooks into using contexts. On my side, I am heavily using supertest and I would love to try to use contexts on passing supertest responses from hooks to tests.

[novemberborn]

@kugtong33 a recipe would be good. It's too much to capture in the README itself.