Updated to use CTools plugins to provide content and configuration for subpages. This version includes the same functionality as 1.x, but the underlying code was largely re-written.
If upgrading from 1.x: Make sure that CTools is enabled, and run the update process. Any existing subpage configuration will be converted to the new format in the database, and continue working as it did before.
This is being marked as beta because I have only tested it myself. I would love to get some feedback from anyone else who tries it out.