default/index subpaths

[jesseditson]

• Add a default reserved property to add to objects with templates
• If an object definition has a "default", copy that page as the index.

Solves for the currently awkward situation that if you use a template (e.g. posts), visiting /posts will 404 - only /posts/post-name will work. There should be a simple way to define a root for these folders.