Implement 'Query for supported vocabulary'

To allow clients to understand what's supported by this server, we can return the list of post types that are supported by the server.

We can generate this automagically based on the Kinds implemented, so this will advertise new types of post as they continue to be created.

As per my proposal on https://github.com/indieweb/micropub-extensions/issues/33, I want to note which properties are required, so a client can provide a hint to the user. Instead of providing one key for required and one for optional, we can instead just return the union of the two, and the required ones.

I've also decided to implement this as a standalone query.

Closes #198 (closed), #199 (closed).