New domains plugin and nginx extension

- New domains plugin that follows the heroku domains plugin api
- Fix NO_VHOST check
- Move nginx configuration specification to templates
- Allow overriding nginx template with a custom one in $APP/nginx.conf.template
- Unify VHOST file creation

TODO:

- [x] Update the URL file with all hosts
- [x] Ensure all new files are backed up properly
- [ ] Show http and https urls in deploy. Currently just shows whatever is in VHOST without a scheme. Should at least show the scheme.
- [ ] Add a `CANONICAL_HOST ` config var that all vhosts should redirect to if available
Nope. That would be an issue. `dokku help` works but help for the specific plugin does not. This is true for all plugins. Also `domains:help` is the only one that enumerates itself. I think we should just remove that for now.