added detection for a legacy content-type: `application/x-genesis-rom`

http://freedesktop.org/wiki/Software/shared-mime-info/
  The `shared-mime-info` package contains the core database of common types.
  It's common among linux distributions.

https://bugs.freedesktop.org/show_bug.cgi?id=27441
  bug: `.md` files should be plain text documents

http://cgit.freedesktop.org/xdg/shared-mime-info/commit/?id=eb5f17c1ae435ab4aaf267b9373903be05448f94
  commit made on: 2011-06-21
  changed the mime-type associated with the file extension `.md`
  from `application/x-genesis-rom` to `text/x-markdown`