Asynchronously init choo stores before list routes

[tornqvist]

This is a 🙋 feature

Checklist

• tests pass

Context

This allows for asynchronously registering routes using choo stores. There's been a lot of talk lately about async render in choo (choojs/choo#646) and I believe we should look into async initialization at the same time. This PR would allow us to start experimenting with that without having to commit to breaking changes in the API.

Here's an example choo store which, when registered, adds a promise to the _experimental_prefetch array which resolves once it has fetched all posts from some CMS and added them as routes to the app. These routes are included in static builds and enjoy proper SSR.

module.exports = store

function store (state, emitter, app) {
  state._experimental_prefetch.push(
    getPostsFromCMS().then(function (posts) {
      posts.forEach(function (post) {
        // add route for each post
        app.route(`/posts/${post.slug}`, require('../views/post'))
      })
    })
  )
}

This depends on choojs/choo#649 which allows for custom state to be passed into stores.

Semver Changes

Minor

[goto-bus-stop]

gotta bump this up my queue. i should get to it thursday morning!

[goto-bus-stop]

lol sorry for ignoring this so long. the code LGTM, but the Choo 7 upgrade introduced some merge conflicts.

[tornqvist]

Ohh, I had forgotten about this. I'll try and resolve the conflicts sometime later this week.