Pre-populate Vuex during SSR
Currently there is no elegant way to pre-populate Vuex store *globally*. The [documentation](https://ream.js.org/guide/data-prefetching.html#prefetch-data-per-component) only provides example of how to run an action from a component's `getInitialData`. Vuex is however by its nature a *global* store which is not logically coupled with a specific component.

In Nuxt, a hard-coded action `nuxtServerInit` is called during SSR: https://nuxtjs.org/guide/vuex-store/

I think Ream should either do the same, or support optional `initStore` function returned from the entry (and called by the Vuex plugin), or even both.

For now, I am doing this in the entry file:

```js
export default function () {
  const store = createStore()
  return {
    router: createRouter(),
    store,
    async middleware (context) {
      if (context.req) {
        // req is only present on the server
        // pre-populate Vuex store
        await initStore(store, context)
      }
    },
  }
}
```

which works but is needlessly low-level code (I am not even saying that `middleware` is not documented and I just stumbled upon it during reverse engineering).