Added domains plugin for working with VHOST files

As discussed in #263, this plugin adds commands to `dokku` to modify app VHOST files in a Heroku-like manner.

```sh
dokku domains [appname] - list all domain names
dokku domains:help
dokku domains:add [appname] DOMAIN
dokku domains:clear [appname]
dokku domains:remove [appname] DOMAIN
```