Added "preview" option to MarkdownDeepEditorUI

This pull request adds a "preview" option to the MarkdownDeepEditorUI javascript class. I have a use case scenario where I don't want to render the preview div, and it's a non-breaking change so I think it is a good addition.

Please note that I used Web Essentials to minify the scripts, because I am unfamiliar with MiniME.
If you don't like that, may I ask that you take care of just taking the changes made to MarkdownDeepEditorUI.js file and minifying and updating the lib file? Just this once? :)