allow exporting a more complete version of feeds

[mockdeep]

The base OPML format includes only the feeds, not unread stories or stars. This appears to be the standard for feed exports, but we may want to have a more complete export format available. It might be worth looking into exports provided by other feed readers to get a sense of what options they provide. Allowing people to export will make it easier to transition between instances when upgrading or switching VPS providers without needing to manage database dumps.