Bundle casks in external Caskfile

(I noticed your setup script when I looked for forks of Homebrew Cask that could cause your [issue with OneNote](https://github.com/caskroom/homebrew-cask/issues/5394).)

This is just a suggestion, of course, but you might want to keep your cask list outside of the bootstrap script. The native `brew bundle` command makes it quite easy, and arguably more convenient.

For reference, see Homebrew Cask's [automation docs](https://github.com/caskroom/homebrew-cask/blob/master/doc/AUTOMATION.md).