Added label_modifier option on ReferenceField conversion

[sebbekarlsson]

So I had a problem where <select> lists had a fixed label.
For example when I had a ReferenceField(User) on my "EditTransactionForm" the list looked like this:

It was just a list of "User Object" , but I wanted it to display User.firstname in the list instead. This pull-> request makes that possible.

So, now I can do this:

class User(db.Document):
    name = db.StringField()


class Message(db.Document):
    recipent = db.SelectField(db.ReferenceField(User))

    def recipent_label_modifier(obj):
        return obj.title


user_items = [
    User(name=name).save()
    for name in ['Ronald', 'John']
]

Contacts(
    users=user_items 
).save()

ContactsForm = model_form(Contacts)
form = ContactsForm()

And achieve this:

[insspb]

@sebbekarlsson I just diving into flask-mongoengine.
This is pretty old PR, but if you can rebase it to current source code and make some tests (pytest supported) it will be very helpful.

[sebbekarlsson]

@insspb Okay cool! I will look into it soon.

[sebbekarlsson]

@insspb I have been rebasing, wrote some tests, made some modifications. Have a look

[insspb]

@sebbekarlsson Thank you for contribution. Merged in master.