Escape backticks within fenced code blocks

[whichxjy]

If we process Markdown text like this:

~~~
this code block has tildes,
```
and isn't closed by backticks
```
~~~

We will get something like this:

<pre>this code block has tildes,
<code>and isn't closed by backticks</code>
</pre>

But what we expect is:

<pre>this code block has tildes,
```
and isn't closed by backticks
```
</pre>

This is a known issue in bikeshed, which is caused by handling code spans after parsing Markdown.