Escape all elements inside pre tags

This allows the use of a code tag inside a pre tag without introducing errant whitespace.

Why? I have some Jade markup that looks something like this:

```jade
pre
  code.
    foo
    bar
    baz
```

Because I've used a `code` tag instead of just using a `pre` tag, the resulting HTML has indentation which is then reflected on screen. If I use a `pre` tag on its own, the HTML has no indentation.

I'm using `code` inside `pre` because I'm using [Prism](https://github.com/LeaVerou/prism), which requires the use of the `code` tag, based on the [W3C's recommended way of marking up code snippets](http://www.w3.org/TR/html5/grouping-content.html#the-pre-element).

I've included a test case. Please let me know if you need anything else, or if you'd like me to make any changes.