Encapsulated HTML rendering of Markdown source in an API

This patch introduces an API for rendering the content of the file as HTML. The API consists of the interface `Renderable` which is available from the lookup of a Markdown data object.

The API encapsulates getting the file content, creating the PegDown parser and access to the global Markdown options. API users can specify several options to

* select whether the content of the editor (if available) is preferred over the content on disk
* convert relative image paths to absolute URLs
* convert relative links to absolute URLs
* use a simplified HTML template to create Swing compatible output

This patch also updates the preview pane, preview action and export action to make use of the new API.