text: Title: Querying based on value of ReferenceField
  Body: I have a model defined like so:
  ```
  from mongoengine import Document, StringField, ReferenceField
  
  class Bar(Document):
      pass
  
  class Foo(Document):
      parent = ReferenceField('Bar')
      name = StringField(default='')
  ```
  
  If I set up my schema like this:
  ```
      from graphene_mongo import MongoengineConnectionField, MongoengineObjectType
      from .models import (Foo as FooModel, Bar as BarModel)
  
      class Bar(MongoengineObjectType):
          class Meta:
              model = BarModel
              interfaces = (Node,)
  
      class Foo(MongoengineObjectType):
          class Meta:
              model = FooModel
              interfaces = (Node,)
  
  
      class Query(graphene.ObjectType):
          node = Node.Field()
          foos = MongoengineConnectionField(Foo)
          bars = MongoengineConnectionField(Bar)
  
      schema = graphene.Schema(query=Query, types=[Foo, Bar])
  ```
  
  Should I be able to query for `Foo` with `parent` equal to the id of a `Bar`? Example query:
  
  ```
  { 
    foos(parent: "some_valid_id_in_string_form") {
      edges {
        node {
          name
        }
      }
    }
  }
  ```
  