Added option for extensionless URLs

New property `uri.noExtension` allows the user to specify a root folder for which "extensionless" URLs will be created. In practice, the file name is used as a folder and the actual file is generated as index.html inside that folder. `uri.noExtension` defaults to `false`, which does nothing.

For example:

If `uri.noExtension` is set to `/blog`, then after baking, the file `/blog/2014/03/26/post.md` would become `/output/blog/2014/03/26/post/index.html` and the URL would be `<DOMAIN>/blog/2014/03/26/post`.