It would be nice to allow the parsing of extended JSON in the new go driver. Something similar to the `bson.UnmarshalJSON()` function in the *mgo driver*

for example, allow expression like this to be correctly parsed to BSON:

{ "_id": ObjectId("59b7887b90afeae466d0cdcb") }

currently, we have to use `$oid` instead:

{ "_id": {
    "$oid": "59b7887b90afeae466d0cdcb"}
}