Added option to disable pretty URLs

No idea if you're interested in merging this in, but I added this option because I wanted to use this plugin for indexing media content (i.e. not just html and markdown). 

Basically, if `prettyUrls` is set to false, files won't be renamed to `name/index.html` and will keep their original filename . If prettyUrls is set to a file extension, like `.md`, then only files ending in that extension will be renamed to `name/index.html`.