Add CNAME option

When a repo have a CNAME file, it will use the URL on it to map the server_name on ngnix
Merge pull request #657 from arthurnn/CNAME_option

Add CNAME option
change buildstep to my own version
move post-deploy hook

Write $DOKKU_ROOT/$APP/URL before post-deploy hook.
Merge branch 'fix/hostname-url' into develop

* fix/hostname-url:
  move post-deploy hook
Merge branch 'master' into develop

* master:
  Add CNAME option