Add exceptions for HTML5 sectioning tags

[darkfeline]

I apologize in advance for my lack of knowledge of blackfriday and HTML, but there seems to be a lot of sectioning/block level tags that need special escaping, such as:

<section>
<article>
<nav>
<address>
<aside>
<header>
<footer>

For example, if I have the following Markdown:

<nav>Stuff in here</nav>

blackfriday will happily wrap them in <p> tags, resulting in invalid HTML5:

<p><nav>Stuff in here</nav></p>

I came across this via Hugo, so I apologize if this isn't a bug in blackfriday.