Added support for custom messaging on a per field/rule basis.

Modified `FormValidator.prototype._validateField()` to support allowing custom messaging on a per field/rule basis. For instance, you can now use the following syntax to target applying a unique validation error messaging to the field `fieldName` when it fails `required`:

```js
validator.setMessage('fieldName.required', 'This field is required, yo');
```

The format is `fieldName`.`validationRule` when using `setMessage()`.

This scratches my itch for overriding messaging on a low level. Hope others find this helpful!