How can I get a running total with mongodb aggregate framework?

I am fairly new to MongoDB and I am playing with the aggregate framework. One of the examples from the documentation shows the following, which returns total number of new user joins per month and lists the month joined:

db.users.aggregate(
  [
    { $project : { month_joined : { $month : "$joined" } } } ,
    { $group : { _id : {month_joined:"$month_joined"} , number : { $sum : 1 } } },
    { $sort : { "_id.month_joined" : 1 } }
  ]
)

The code outputs the following:

{
  "_id" : {
    "month_joined" : 1
  },
  "number" : 3
},
{
  "_id" : {
    "month_joined" : 2
  },
  "number" : 9
},
{
  "_id" : {
    "month_joined" : 3
  },
  "number" : 5
}

Is it possible to also have each object contain the sum of all users that have joined since the start, so I don't have to run over the objects programmatically and calculate it myself?

Example desired output:

{
  "_id" : {
    "month_joined" : 1
  },
  "number" : 3,
  "total": 3
},
{
  "_id" : {
    "month_joined" : 2
  },
  "number" : 9,
  "total": 12
},
{
  "_id" : {
    "month_joined" : 3
  },
  "number" : 5,
  "total": 17
}