Added domains plugin which implements `dokku domains` commands for
working with VHOST files.
Pulled automatic subdomain function from nginx-vhost to separate plugin.

All functionality under the old nginx-vhost has been preserved. Helper
messages have been added and commented parts of code.
Added ability to deploy to multiple custom domains in VHOST.

- nginx-vhost plugin now no longer manages `/home/git/$APP/VHOST`
  file but rather reads from it.
- nginx-vhost plugin supports multiple custom domains in VHOST.
- Default nginx configuration template pulled out into separate
  files instead of adding as a HEREDOC in `post-deploy`.
- Added helper messages during deploy.
- Minor code cleanup.
Merge branch 'subdomain'