title: GraphQL: Filter data in an array
body: <p>I'm sure it's a simple thing to do, but I couldn't find anything in either GraphQL's doc or Graphcool's.</p>
  
  <p>Say I have an entity with this schema (new GraphQL user, sorry if I make mistake in the schema representation):</p>
  
  <pre><code>Book {
    name: String!
    author: String!
    categories: [String!]
  }
  </code></pre>
  
  <p>How would I do a query for all books that are part of the <code>"mystery"</code> category? I know I can filter with <code>allBooks(filter: {})</code>, but <code>categories_in: ["mystery"]</code> and <code>categories_contains: "mystery"</code> didn't do the trick.</p>