Add support for Posts in Markdown format

I was just doing this on my own fork but figured I'd submit a PR in case you're interested in this.

I've added support for posts to be in Markdown format as well (`.md` extension). For Markdown posts, I changed the "meta" block from `<script></script>` to `---` which matches Jekyll (HTML posts remain as is).

edit: If you are interested, I'll update the `README` to reflect these changes as well.