Escapes in inline links

Reference implementation allows escaping with \ in inline URLs, e.g. from one of its unit tests:

```markdown
*[closed](hello\*)
```
```html
<p>*<a href="hello*">closed</a></p>
```

jotdown currently does not:

```html
<p>*<a href="hello\*">closed</a></p>
```

also useful for e.g. URLs with parenthesis.