[LAB-10] Add HTTPS support for ZAT

:koala:

Add HTTPS support for ZAT

```
$ zat serve --ssl-cert /opt/boxen/config/nginx/ssl/zendesk.dev.crt --ssl-key
/opt/boxen/config/nginx/ssl/zendesk.dev.key
== Sinatra/1.4.5 has taken the stage on 4567 for development with backup from Thin
Thin web server (v1.6.2 codename Doc Brown)
Maximum connections set to 1024
Listening on localhost:4567, CTRL+C to stop
== ZAT Server is running
You may now start using ZAT by appending ?zat=https://localhost:4567/app.js to your Zendesk URL
^CStopping ...
Stopping ...
== Sinatra has ended his set (crowd applauds)
```

/cc @zendesk/quokka

### References
 - Jira link: https://zendesk.atlassian.net/browse/LAB-10

### Risks
 - None
Could we use the variables defined above instead?