Feature/markdown

Upgrade Markdown vendor to 2013 version (from 2009). The _latest_ version is a composer package so holding off on that til we refactor. 

ALSO: Adds a new default so that line breaks are preserved in Markdown. I've gotten a lot of feedback that this is the desired outcome for forum discussion and disappearing line breaks is an unwelcome surprise for most casual users. I cannot think of a scenario where the no-breaks behavior would be something folks rely on in forum commenting, so making the change.