Fix anchors containing block elements

[Ndpnt]

Fixes #409

[aaronmfparr]

This is a very simple fix and solves my problems, so I am all for a merge here.

What's the hold up?

[domchristie]

Unfortunately this change doesn't result in the correct markdown. For example, the following HTML:

<a href="http://example.com/heading"><h1>heading</h1></a>

gets converted to:

[heading
=======](http://example.com/heading)

and so when converted back to HTML is:

<p><a href="http://example.com/heading">heading
=======</a></p>

This is quite a complex issue, and becomes trickier when multiple block elements are wrapped in a <a> (see #313 (comment)). Ideally the HTML should be changed so that the <a> only wraps text. So currently your best bet is to mutate the HTML to fix these issues before passing it in to Turndown.

[aaronmfparr]

thanks for responding to this. hopefully we'll see some movement.