Markdown implementation?

Adds markdown parsing to `post.content` post db retrieval. Not sure this is even in the plans for BBS, but thought it a trick addition. Feel free to flame me if it's terrible ;)

**Warning**
Untested stuff here. Have to run to the store, but will be back later.
Right here, the `sanitize: true` prohibits arbitrary markup from passing through. That should absolve your concern for XSS.