feat: support config files exporting (`async`) `function`s#10001

Merged

SimenB merged 4 commits intojestjs:masterfrom hisco:async-config

May 9, 2020

+56-2

Commits on May 7, 2020

Updated doc to support async
hisco
committed

Commits on May 8, 2020

Added unit tests async config
hisco
committed

Commits on May 9, 2020