Global webhook URL not returned if querying a single app

[matt-oakes]

If you have a global webhook URL (new from #6) set but not one an app and then ask for that apps webhook URL it should return the global URL. Currently:

$ dokku slack:set https://hooks.slack.com/global
$ dokku slack:get
https://hooks.slack.com/global
$ dokku slack:get an-app
cat: /home/dokku/an-app/SLACK_URL: No such file or directory
$ dokku slack:set an-app https://hooks.slack.com/single
$ dokku slack:get an-app
https://hooks.slack.com/single

Should be:

$ dokku slack:set https://hooks.slack.com/global
$ dokku slack:get
https://hooks.slack.com/global
$ dokku slack:get an-app
https://hooks.slack.com/global
$ dokku slack:set an-app https://hooks.slack.com/single
$ dokku slack:get an-app
https://hooks.slack.com/single