add server- and client-side store management (#178)

[Rich-Harris]

This addresses the first piece of the #178 puzzle: an idiomatic way to assign a store to each individual request on the server, and to rehydrate it on the client:

// app/server.js
import { Store } from 'svelte/store.js';
// ...

app.use(sapper({
  routes,
  store: req => {
    return new Store({
      user: req.session.passport && req.session.passport.user
    });
  }
}));

// app/client
import { init } from 'sapper/runtime.js';
import { Store } from 'svelte/store.js';
import { routes } from './manifest/client.js';

init(target, routes, {
  store: data => new Store(data)
});

With this change, any of your components could reference e.g. {{$user.displayName}}.

The server-side store can contain any data that can be serialized with devalue, otherwise on the client data will be undefined.