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