Allow property names to be used for format validation

String representations of property names that return regular expressions
will be accepted by the format validation. For instance, if a model has
a property, `attributeFormatter`, which returns `/\d+/`, then the
following validation syntax is legitimate:

```javascript
validations: {
  attribute: {
    format: {
      with: 'attributeFormatter'
    }
  }
}
```

My particular use-case requires format validations to be dependent on some other model property. For example, validating zip code formats would depend on a country or region.