support non-ruby-backed casks

- brew cask list now displays casks without backing ruby files
- casks without a source are displayed as "caskname (!)"
- these casks can be uninstalled, with the caveat that it only removes
  their files from the caskroom (doesn't run pkg uninstall or anything,
  since there's no ruby file to define what to do)
Merge pull request #1948 from phinze/non-ruby-backed-casks

support non-ruby-backed casks