Reference with multiple node types

[hjvedvik]

This PR makes it possible to reference multiple node types. Also adds sortBy, order, skip and limit arguments for references which returns a list of nodes.

Store API

books.addNode({
  title: 'Title',
  fields: {
    shops: [
      { typeName: 'PhysicalShop', id: '1' },
      { typeName: 'OnlineShop', id: '1' }
    ]
  }
})

Example query

query Book ($id: String!) {
  book (id: $id) {
    title
    shops (sortBy: "title", limit: 5) {
      __typename
      ...on NodeInterface {
        title
      }
      ...on PhysicalShop {
        address
      }
      ...on OnlineShop {
        website
      }
    }
  }
}