[feature] enable config overrides

[gabrielcsapo]

fixes #38

Summary

Allows config overrides via existing config object such as

"storybook": {
  "config": { 
    "script": {
      "src": "./assets/foo.js"
    }
  }
}

Other

I migrated the tests to Jest as the manual string assertion makes it hard to add tests.

[gabrielcsapo]

I am going to add husky and lint-staged in the next PR. Lots of good cleanups we can do after almost 3 years of the initial implementation... time flies.