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

- [ ] Add tests
- [ ] Get feedback on feature
- [ ] Evaluate need for an app-level augment
- [ ] Clean-up implementation