#12 Custom stylesheet support. 

Also added a Darcula themed stylesheet and the ability to pick between the default and dark style.

The pattern that I chose for the settings panel is a bit different from what is currently in the project: I chose to wrap the logic of the several different UI components within the settings panel and keep in encapsulated there.

One additional todo would be to update the Markdown preview panel when the user changes the settings of the plugin on the fly.