- Collections always render as directories, to allow nesting
- Collections monkey-patch get_collections and get_collection helpers
  onto all bundles
- Bundle filtering sorts with dates auto-converted to datetime, to
  compare together
Allow output directory to be specified in settings for a site.
Allow sorting of bundles by a comma separated list of properties, not just one.
When processing content types, stop when we find one.
Allow bundles to define a url override.