Multiline Link Reference Definitions are parsed incorrectly.

The CommonMark spec allows link reference defintions to span multiple lines.

For example, the [spec states that the following markdown](https://spec.commonmark.org/0.29/#example-164):

```markdown
[Foo bar]:

'title'

[Foo bar]
```
should render like this:

```html

Foo bar

But this library fails to recognize the link definition and instead renders it like this:

```html

[Foo bar]:

'title'

[Foo bar]