Support posts as index file in post-specific directory (Issue 511)

The attached commits add support for posts named `index.extension` inside a directory which follows the naming convention for posts and contains the post information.  As suggested in Issue #511, this would allow authors to store content associated with the post alongside the post content file to ease the process of managing files in some cases.

Note that these changes do not add any support for copying post content files.  That may be implemented later, or done using plugins (e.g. https://gist.github.com/3131752).