title: Add AllowDiskUse(true) to aggregation
body: <p>I am getting a 'sort exceeded memory limit...' error which dictates using allowDiskUse(true) in my aggregate. My problem is that I can't quite figure out where to add that to my code. I have tried adding it as an object within the pipeline and as a property in the aggregate() method call and i get an error for both at runtime.</p>
  
  <p>Code is below:</p>
  
  <pre><code>server.get('/cheap-flight-by-route', function (req, res, next) {
  
      Flights.aggregate(
          {$sort: {'fare.total_price': 1}},
          {$lookup: {
              from: 'travelroutes',
              localField: 'route',
              foreignField: '_id',
              as: 'routes'
          }},
          {$match: {
              'routes._id': {$exists: true}
          }},
          {$group: {
                  _id: {
                      departureAirport: '$routes.departureAirport',
                      arrivalAirport: '$routes.arrivalAirport',
                  },
                  total_price: {$min: '$fare.total_price'},
                  avg_price: {$avg: '$fare.total_price'},
                  created: {$first: '$created'},
                  doc: {$first: '$$ROOT'}
              }
          },
          {$project: {
              departureAirport: {$arrayElemAt: ['$_id.departureAirport', 0]},
              arrivalAirport: {$arrayElemAt: ['$_id.arrivalAirport', 0]},
              created : '$created',
              price: '$total_price',
              averagePrice: '$avg_price',
              'doc': 1,
              '_id': 0
          }},
          {$sort: {
              'created': 1,
              'departureAirport': 1,
              'arrivalAirport': 1
              },
          },
          function(err, cheapFlights){
              if (err) {
                  log.error(err)
                  return next(new errors.InvalidContentError(err.errors.name.message))
              }
              res.send(cheapFlights)
              next()
          }
      )  // &lt;-- if I add a .allowDiskUse(true) here it throws a 'bad property' error
  })
  </code></pre>