Preshow should ignore files we know aren't images
Preshow should ignore files we know aren't images

This allows the author to add a `README.txt` or `.md` to describe what the directory is for.
puppetlabs/showoff,marrero984/showoff,marrero984/showoff,marrero984/showoff,puppetlabs/showoff,binford2k/showoff,marrero984/showoff,puppetlabs/showoff,binford2k/showoff,binford2k/showoff,puppetlabs/showoff,binford2k/showoff