Add CommonJS app

You can now break your app into CommonJS modules, which can be then `require`d in your app. Modules will be automatically loaded from the '/lib' directory and are made available in your app by using `require('filename.js')`.

/cc @zendesk/quokka

### REFERENCES
- [JIRA](https://zendesk.atlassian.net/browse/APPS-1524)

### RISKS
- None