fallback to reference if a link is not valid
fallback to reference if a link is not valid

This commit makes martdown-it pass a new example introduced in https://github.com/jgm/CommonMark/commit/cfc84164475d3bec8be9482c21a705adc93a54f5

```
[foo](not a link)

[foo]: /url1
.
<p><a href="/url1">foo</a>(not a link)</p>
```

Ref: https://github.com/jgm/CommonMark/issues/427

rlidwka/markdown-it,markdown-it/markdown-it,GerHobbelt/markdown-it,GerHobbelt/markdown-it