Feature request: Add support for link (and image) attributes

With issues #116 and #128 going in, we'll have support for titles on links. But there are times when I need to add other attributes such as class, rel and others to links. I know the original markdown syntax doesn't support something like this, but without it, I'm forced to drop down to writing literal HTML for links in most cases. The same is true for images as I want to add the class attribute for them.

I'm proposing the following. It's not my idea, but I like how little it impacts the markdown syntax in terms of aesthetics and readability.

This is a formatted ![penguin_image][] image and a [penguin_link][] link with attributes.

[penguin_image]: http://foo.com/penguin.jpg "penguin" class="img-responsive center-block"
[penguin_link]: http://foo.com/penguin.html "Penguins" rel="external nofollow"
                style="color: red;"