Document tags stanza

There will be no mass-add of `tags` to Casks (at least not yet) as I don't have anything accurate.

This PR adds `tags` to two Casks for use as examples.

Cask authors are encouraged to fill in accurate values for `:name`/`:vendor` tags as soon as this PR is merged.  However, front-end commands such as `brew cask search` do not yet read `tags` values.

Refs: #6354