Parameterise install location

Here are some proposed changes to the `brewcask` class so it fits in better alongside other Boxen modules.

In summary:
* Parameterise the install location of brewcask
* Add in tests for the main `brewcask` class
* Set it to install apps to /Applications by default (based off of [puppet-boxen/manifests/personal.pp](https://github.com/boxen/puppet-boxen/blob/7ef6c67a6dc367ccc2483eeae0504e78a21f5d79/manifests/personal.pp#L60))
* Add in a `brewcask_root` fact (similar to [puppet-homebrew](https://github.com/boxen/puppet-homebrew))
* Add in a `boxen::env_script` that handles the setting of some basic `HOMEBREW_CASK_OPTS`