auto_linker should not link inside <pre> and <code>
When I write a markdown-post like this:
```
192.168.0.0/16
```
This is the result:
<pre>
<code><a href="http://192.168.0.0/16" rel="noopener nofollow noreferrer">192.168.0.0/16</a></code>
</pre>
I'm on commit e19590c9