Unify doc explorer navigation around schema reference concept

As defined in https://github.com/graphql/codemirror-graphql/blob/master/src/utils/SchemaReference.js - SchemaReference gives us a serializable way to refer to different parts of a schema.

Ideally this general utility is moved into a place where it can be leveraged more broadly (graphql-js perhaps) along with serialization and deserialization utilities.

This would make a better general public API for navigating to any element, and would give codemirror-graphql a better interface for interoperating with any kind of documentation browser.

It would also unlock issues like #77 by allowing a serialization of the currently viewed doc