[APPS-1567] As an App developer, I can define an author url in my manifest

:koala:

This PR adds a prompt on `zat new` to ask the app author for the associated url. The field is optional and is then added to the generated manifest.json. 

/cc @zendesk/quokka 

### REFERENCES
- [JIRA](https://zendesk.atlassian.net/browse/APPS-1567)
- [ZAS PR](https://github.com/zendesk/zendesk_apps_support/pull/63)
- [ZAF PR](https://github.com/zendesk/zendesk_app_framework/pull/184)

### RISKS
- None