In-layout preview for Jekyll posts

For markdown files with front matter and a defined layout (Jekyll posts) show an alternate preview that renders the post in its layout template (following nested templates) and render includes. Fail gracefully for unavailable template instructions, which is basically anything to do with the `site` object, outside what's specified in `_config.yml`.

This could use some more testing, but should be close. 

![Screen Shot 2013-03-27 at 7 10 52 PM](https://f.cloud.github.com/assets/170641/311158/9bb9be42-9733-11e2-9fc9-758d2960cec0.png)

![Screen Shot 2013-03-27 at 7 10 54 PM](https://f.cloud.github.com/assets/170641/311159/9ff12de2-9733-11e2-8847-e1da75db053d.png)



cc @tristen @mikemorris