Attributes not supported in image references

The following snippet correctly applies the `id` attribute to the `<img>` tag:

```
![image{@id=foo}](http://example.com/i.jpg)
```

When using image references, the same translation is not applied:

```
![image{@id=foo}][image]

  [image]: http://example.com/i.jpg
```

The above results in `alt="image{@id=foo}"`.

For consistency, attributes should be supported with image references.
