Tabular code examples

@montypivotal designed this cool interaction where simple components (like typography) can be laid out horizontally with their code snippet and a copy button that copies the code to the clipboard. You can see his mockup below.

![pui-16](https://cloud.githubusercontent.com/assets/824157/4493291/391b7356-4a48-11e4-80e7-c378701d21bd.png)

This is the first pull request toward making this possible. It allows you to indicate component examples that should be laid out in a table via the `html_example_table` and `haml_example_table` identifiers.

We'll have a followup commit on the [hologram-example repo](https://github.com/trulia/hologram-example) that adds more docs about how to use this feature. We'll also followup in a separate commit with the copy-button functionality added via JS.

What do you think? Any suggestions for improvement?

Geoff & @stubbornella 
I think this require is better placed in markdown_renderer, since that is the only thing that needs it right?