Add Jest configuration for snapshotresolver

Created by: akuji1993

As I understand, right now CRA only supports a handful of Jest configuration overrides, either by adding them to package.json or adding a jest.config.js to your project.

First question would be, why are you not allowing to just add a jest.config.js and read all of the configuration variables that Jest provides usually, why add just a few of them?

And the more important suggestion, can you add snapshotresolver to this list of allowed customizations? https://jestjs.io/docs/en/configuration#snapshotresolver-string

It has been added with Jest 24 and would really help clean up the codebase a bit, since I don't think it's necessary to have generated code right next to my .tsx, .scss and .test.ts files.. If not, could you at least explain, how the decision was made to not just check for a jest.config.js and then give it to the test runner, so the dev can decide what they want to override or not.