Add a "tip updated date" to tip files #49

When we update a tip, we usually update its date too. But doing so means moving it right back at the top of the list, both on the site and in the RSS feed.

This isn't always wanted. I think we should have both a date (to sort the tips on the site and feed), and a "updatedOn" date that's just for users to know when a tip's content was last updated.