code: function buildSlug(data, update, done) {
      var opt = buildOptions(data)
  
      // store old slug
      if (update) {
        _.get(data, slugsFieldName).push(_.get(data, slugFieldName))
      }
  
      // Get next counter within scope
      Counters.findByIdAndUpdate(
        opt.key,
        { $inc: { seq: 1 } },
        { upsert: true, new: true },
        function(err, res) {
          if (err) {
            throw err
          }
  
          var seq = res.seq,
            slg = opt.slg
  
          // If found more than one, then add suffix to the slug
          if (seq > 1) {
            slg = slg + '-' + seq
          }
  
          // store new slug
          _.set(data, slugFieldName, slg)
  
          done(null, slg)
        }
      )
    }
docs: Build slug