Switch to Pegdown Markdown processor from MarkdownJ

Hi, I'm wondering if you'd consider a switch to the Pegdown Markdown processor.  It has support for some very useful Markdown extensions (mostly those from GitHub and PHP).  MarkdownJ has plans to support extensions, but doesn't currently.  I've switched to Pegdown in this pull request and created some tests for the extra features (in the process, splitting the Markdown parser tests out into their own file).  Thanks for your consideration and thanks for creating jbake.