Expose subscription client

[maclockard]

This makes it possible to manually set callbacks for onDisconnected, onReconnected etc. The advantage here is end users will be able to better handle network blips rather than having apollo have to handle it. See https://github.com/apollographql/apollo-link/issues/234 for an example of folks using this functionality now.

An alternative implementation would look like exposing these callbacks as optional parameters in the constructor. Let me know what you prefer!

TODO:

• [ ] Make sure all of new logic is covered by tests and passes linting
• [ ] Update CHANGELOG.md with your change