Options can be passed on the command-line and/or using
the HOMEBREW_CASK_OPTS environment variable (which has
lowest priority). There is a single --appdir=PATH option
right now, but this commit enables future awesomeness!
Other minor changes:
* `brew cask help` now returns the same thing as `brew cask`
instead of saying there was “no such command as help”.
* The HEREDOC block now uses Homebrew's #undent instead of the
customed-rolled #gsub version. Cleaner and more flexible.
* `Cask.set_appdir` has been renamed to `Cask.appdir=`. This
is more Rubyish, and of little consequence (the only place
it was previously used was in the tests).
phinze