Added 'useRelativeDirPath' config option

Added a `useRelativeDirPath` config option that appends the document's relative directory path to the associated files path.

This allows you to organise your associated files in a more structured way, and one which mirrors the document the files are associated with.  For example, if your document lives at `/documents/posts/2012/post1.html.md` and `useRelativeDirPath` is set to true, then the plugin will look for associated files at `/files/associated-files/posts/2012/post1/` rather than `/files/associated-files/post1/`.