Non-eject customization

[shilman]

Currently, to set up API hooks, you need to create a custom jest config.

Here's a strawman API for adding the API hooks that doesn't require ejecting:

// .storybook/test-runner.js
module.exports = {
  hooks: {
    preRender: (page, context) => { ... },
    postRender: (page, context) => { ... }
  },
}

Notes:

• This allows us to change the test runner more easily in the future
• This uses the .storybook directory, and maybe we should consider that for the jest config also

// jest-config.js
try {
  const { hooks } = require('.storybook/test-runner');
  ..
} catch(err) {}

[shilman]

implemented a slightly different design in #53