Markdown preview support 

Would be nice to have markdown preview support. This PR is just a sample test run using dialog box. Spark has grown so much I need some help identifying best place to render html to. Was thinking of using the chrome library support to launch a new window. 

![screen shot 2014-05-21 at 9 48 18 pm](https://cloud.githubusercontent.com/assets/654526/3049952/f485050c-e16c-11e3-9f4f-e0cbf48168f3.png)