accepts a single argument, which is a relative path to a pkg
inside the extracted Cask; homebrew-cask will attempt to install this
pkg after the Cask is extracted via `installer`
because of the many different ways uninstallers work, this
has several features:
- `:script`: a script in the Cask which serves as an uninstaller (e.g.
Vagrant, VirtualBox), uses `:args`, and `:input` keys to interact
with said script
- `:pkgutil`: a regexp which captures all package_ids installed by this
cask; homebrew-cask will list all files installed under these ids and
remove them
- `:launchctl`: a list of bundle_ids for services that should be
removed by homebrew-cask
- `:files`: a fallback list of files to manually remove; helps when
uninstallers miss something
refs #661
