Is there any way to set a field with an "unmodifiable" setting (Such as type, required, etc.) when you define a new Mongoose Schema? This means that once a new document is created, this field can't be changed. 

For example, something like this:

```
var userSchema = new mongoose.Schema({
  username: {
    type: String,
    required: true,
    unmodifiable: true
  }
})

```