Provide a public real-time API for subscribing to annotation updates

## Feature Request Form
### Problem you are trying to address with this feature

Provide a way for third party developers to get real-time notifications when annotations are added to, updated or removed from a document. See [Slack discussion](https://hypothes-is.slack.com/archives/public/p1489217752364808) for the origins of this request.

The Hypothesis client uses the [Real Time API](https://h.readthedocs.io/en/latest/realtime/) for this, but the protocol for subscribing to annotation updates for a URL is a) not documented and b) not consistent with the APIs that are documented.

### Your solution

1. Design an API for a client to indicate the URLs that it is interested in receiving annotation updates for and receiving those updates. This API should be consistent with [the protocol used by the examples](https://h.readthedocs.io/en/latest/realtime/#server-messages).
2. Implement this API and modify the client to use it instead of the current undocumented API.
3. Document the API on [RTD](https://h.readthedocs.io/en/latest/realtime/#server-messages)

Start with the Real Time API server-messages documentation and the protocol used by its examples, then trace how the Hypothesis client currently subscribes to annotation updates. Define a consistent public subscription protocol, update the client to use it, and document the completed API on RTD.

Written by the indexing model from the issue text.