Allow configuring reporters for Jest cont.

Created by: TomasHubelbauer

I'd like to suggest that reporters be configurable through the jest object in package.json. This was suggested in #2474 (closed) and a workaround was presented which works great, however no word was said on whether it is up for debate to include the reporters in the set of "blessed" keys that we can set through package.json. I am interested in how those were chosen and if the set can be extended. Additionally, a suggestion was made somewhere to just merge the user object and the CRA object, which to me sounds like a better solution that a whitelist. The previous thread was locked down by a bot without a human explaining why so I am opening a new issue so that the communication on the topic can continue.