Recognize more extensions for Markdown files; embed TTF fonts

I made a couple of very small changes, so RPub will recognize the same set of Markdown file extensions as Fletcher Penney's Textmate bundle for MultiMarkdown does. I also included TTF fonts in the embedded assets.

I think there is a minor error in the Readme, which I haven't corrected in case I'm wrong. Your first example config uses `author`, but the spec later on (and Dublin Core) says it should be `creator`.

Nigel Chapman