Markdown Syntax test

Issue Summary

I am using Markdown as my only filter. I've done a syntax test based on the Markdown syntax documentation and the results don't exactly match what it "should" be. In particular, [src] doesn't seem to work, and there's something that's causing half the document to be bold.

Note: there's a portion of that document that contains echo $input | $markdown_script that will freak out Drupal. You'll need to remove that to accurately test it.