Add markdown hints

This adds a mode that shows hints of the markdown that was used to create the tags. This is a really useful way to teach people markdown.

![markdown-hints](https://f.cloud.github.com/assets/173/2119029/fd684da0-913a-11e3-9dec-16fc4da8303e.gif)

It is all just generated with `:before` and `:after` selectors on the elements.