Populated Virtuals Setter Feature Request

This is a **feature request**. I am using Mongoose version **4.11.11**.

I recently discovered populating virtuals which allows us to use foreign keys other than `_id`. In my opinion, this is a really great feature. However, I have one small suggestion to improve its functionality.

Currently, the virtual field cannot be set directly. Since the virtual field should know enough about the the populated version of the document, we can update the virtual field _and_ populate the `localField` for the document automatically.

## Expected Behavior

Here's a short sample of what I think should happen:

```javascript
const mongoose = require('mongoose');

const ReferenceSchema = new mongoose.Schema({
code: String
});

const ReferrerSchema = new mongoose.Schema({
reference: String
});

ReferrerSchema.virtual('vField', {
ref: 'Reference',
localField: 'reference',
foreignField: 'code',
justOne: true
});

const Reference = mongoose.model('Reference', ReferenceSchema);
const Referrer = mongoose.model('Referrer', ReferrerSchema);

const referrer = new Referrer({
vField: new Reference({
code: 'myCode'
})
});

referrer.reference.should.equal('myCode');
referrer.vField.code.should.equal('myCode');

referrer.vField = new Reference({
code: 'myNewCode'
});

referrer.reference.should.equal('myNewCode');
referrer.vField.code.should.equal('myNewCode');
```

## Current Behavior

Currently, setting the virtual field has no affect on either the virtual field or the `localField`.

## Possible Solutions

I haven't been able to examine the code for this type of virtual, but creating a setter similar to below should do the trick:

```javascript
virtualField.set(function(populatedDocument) {
this[localField] = populatedDocument[foreignField];
this._virtualField = populatedDocument; // Assuming the populated document is stored on this as _virtualField
});
```

The issue names no files or tests. Start by tracing how populated virtuals are defined and assigned in Mongoose 4.11.11, then compare that behavior with the provided Referrer/Reference example. Done means assigning vField updates both the virtual value and the reference localField for both initial assignment and reassignment.

Written by the indexing model from the issue text.