Add failing test for HTML with loose angle brackets in content.

Loose open angle brackets (<) within the content (text) of the HTML are being treated as the beginning of opening tags, causing something like
```
<p>
If x < 4, then x^2 < 16
</p>
```
to fail.  

On the one hand, '<' should really have been escaped, so maybe it's beyond the scope of this package.  

On the other hand, almost all browsers treat this situation gracefully, which means that there are oodles of mathy HTML pages full of these loose, unescaped less-than symbols... and this module (or, more to the point, substack/node-trumpet, which relies on it) might actually be the tool I'd want to use to fix some of these kinds of escaping issues in projects that involve handling big swaths of crappy, 3rd party HTML.