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
* 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`

This isn't ready just yet, but it's something to get the ball rolling with :+1: