Override Jest config by package.json

Created by: pgdejardin

The developer can add a Jest config in package.json and override what it wants from the config file generated by react-script

I've made this to answer #1455 (closed) and #1830. I had to ignore *.stories.js from my coverage script and the coverage threshold and I didn't want to add them in the script himself but as Jest config.

This is a config that could be used :

"jest": {
    "collectCoverageFrom": [
      "src/**/*.{js,jsx}",
      "!src/**/*.test.{js,jsx}",
      "!src/**/*.stories.{js,jsx}",
      "!src/stories/**/*.{js,jsx}"
    ],
    "coverageThreshold": {
      "global": {
        "statements": 98,
        "branches": 91,
        "functions": 98,
        "lines": 98
      }
    }
  }

I hope it will help

Created by: pgdejardin

In a second though, adding configuration in package.json isn't in the mind of the project.

I close my PR.