Floating images

I think @chrisalley’s concern about the separation of content and presentation is valid. That’s why I think we should allow adding classes to images instead of specifying their floating behaviour directly (unfortunately he seems to be also against classes).

To add the floatLeft class, ![altText](image.png "title"){.floatLeft} seems like a good syntax to me. Especially since it’s already implemented in Markdown Extra and in light of a generic and consistent syntax for attributes.