[PoC] Add jQuery.migrateWarnCallback feature

The reason why I added this, is that with such callback I can notify some exception tracking service with deprecation messages. For example — notifying [rollbar.com](https://rollbar.com):

```js
$.migrateWarnCallback = function(msg) {
  _rollbar.push({
    level: 'error',
    msg: "JQMIGRATE: " + msg
  });
};
```

Thanks!