Add object type-to-route mapping to index for discoverability #577

To enhance client discovery, we'd like for endpoints to be able to register a global name that will appear in the index. For instance, core will register posts, users, comments, terms. Jetpack may register portfolio.

Currently our index returns a list of routes, but the individual routes don't have much metadata about the type of resource each represents.

The degree to which we recognize these as named content objects is TBD.