Add ability to register initial handlers

[kpervin]

Add ability, like msw-storybook-addon, to initialize handlers at root level:

// preview.js
import { initialize } from 'msw-storybook-addon';

initialize({}, [
  rest.get('/numbers', (req, res, ctx) => {
    return res(ctx.json([1, 2, 3]))
  }),
  rest.get('/strings', (req, res, ctx) => {
    return res(ctx.json(['a', 'b', 'c']))
  }),
])