APPS-1524 As an App developer I expect CommonJS Support in Apps

This PR adds support for loading modules in your app, following the CommonJS pattern. Please refer to the Demo App PR linked below for usage examples.

/cc @zendesk/quokka 

### TASKS
- [x] Add/update tests

### REFERENCES
- [APPS-1524](https://zendesk.atlassian.net/browse/APPS-1524)
- [Demo App PR](https://github.com/zendesk/demo_apps/pull/48)
- [ZAF PR](https://github.com/zendesk/zendesk_app_framework/pull/211)

### RISKS
- Additional bloat could be added to an existing app that already uses a `/lib' directory. The code from these files will be loaded into the source of an app.