code: function getCategories (posts) {
    var categories = posts.reduce(function (categories, post) {
      if (!post.category) return categories;
      return categories.concat(post.category);
    }, []);
  
    return _.unique(categories).sort();
  }
docs: Takes an array `posts` of sorted posts and returns
  a sorted array with all categories
  
  @params {Array} posts
  @returns {Array}