Added newline to end of Markdown output

This hardly deserved it's own PR, but here it is anyway.

Just added a newline to end of Markdown pretty printing; the rest of the styles already include this but for Markdown there is no newline at the end of the output so the R prompt would end up in the middle of the screen after calling `bibliography()`.

The rest of the diff is just vim chomping off extra whitespace (sorry about that!)