feat: allow babelrc in babel-jest transformer

[lee-reinhardt]

Fixes #18

Verified that this fixes the outlined issue by testing with the example project.

Ran yarn add emotion react-emotion babel-plugin-emotion. Defined a styled component in a way that requires the Babel plugin:

const Box = styled.div`
  background-color: green;
`

Added this component to the body of App.tsx.

Ran yarn start. It fails to compile because of missing Babel plugin. Updated config-overrides.js with an injectBabelPlugin() helper for the emotion plugin. Verified app now works.

Ran yarn test. It fails to compile because of missing Babel plugin. Created a root .babelrc file and copy-pasted json config example I used in the docs. Verified test now works.

[strothj]

Thanks again! Published as 2.4.0.

[codepunkt]

This just saved my day. Thanks! 🎉