By using `ajv`, how can I reference `foo` to validate that `bar` has the same value?

```
var Ajv = require('ajv');
var ajv = new Ajv({allErrors: true});

var schema = {
  'properties': {
    'foo': { 'type': 'string', 'minLength': 3 },
    'bar': { 'type': 'string', ###HERE### },
  }
};

data = {
  'foo': 'abc',
  'bar': 'ab',
};

ajv.validate(schema, data);   // <-- should return false, foo !== bar

```

I've tried using (with similar variations):

```
...
'bar': { 'type': 'string', 'format': { '$data': 'foo' } },
...

```

But that didn't work.