Create `extend()` API so any Express module instance can be extended

Previously, only the the Express module instance which is a `peerDependency` of this package would be extended with this package's functionality. This now exports the `extend()` function which allows any Express module instance to be extended.

### Todo

- [x] Add tests
- [x] Get feedback on feature
- [x] Evaluate need for an app-level augment
- [x] Clean-up implementation
- [x] Evaluate whether `augment()` is safe for `resProto`, it might be shared between apps
- [x] Get sign-off from real world app usage