Add caveats to info like homebrew

If a cask contains caveats it will print it out when running `brew cask info`

```sh
$ brew cask info menu-meters
menu-meters: latest
http://www.ragingmenace.com/software/menumeters/
Not installed
https://github.com/phinze/homebrew-cask/commits/master/Casks/menu-meters.rb
==> Caveats
You need to run /opt/homebrew-cask/Caskroom/menu-meters/latest/MenuMeters Installer.app to actually install MenuMetres
```

Test attached