Bracket syntax for links

According to [spec](https://github.github.com/gfm/#example-498).
There is bracket syntax for links which contain whitespaces.

However, cmark-gfm-swift does not handled it correctly.

input:
```[link](</my uri>)```

cmark-gfm's output:
```<p><a href="/my%20uri">link</a></p>```

cmark-gfm-swift's output:
```<p>[link](&lt;/my uri&gt;)</p>```