cask/installer: Add Cask caveats to the end-of-operation summary

- Both formulae and casks can have caveats, but only formulae caveats
  were shown at the end of a bulk install/upgrade/reinstall operation
  via `Homebrew.messages.record_caveats`. This fixes that to show Cask
  caveats too, for consistency (scrolling up all of the
  multi-formulae-and-casks output to see caveats is time-consuming and
  users might miss them).
- In doing this I had to change how `Messages#record_caveats` works
  since the cask name is just a string, not an object.