add `zap` stanza to DSL

The zap functionality here is in working form, but there is not a
corresponding `brew cask zap` command verb, which is to be supplied
in a separate PR.

Like other forward-compatible DSL extensions, it is intentionally
undocumented.

References: #4688
Merge pull request #4869 from rolandwalker/add_zap_stanza

DSL: add `zap` stanza