DSL: add `installer` stanza (WIP)

The purpose is to run an installation script, providing cleaner syntax than
the current method, which is to use `caskroom_only` followed by `after_install`.
(example [adobe-air.rb](https://github.com/caskroom/homebrew-cask/blob/master/Casks/adobe-air.rb)).

This PR is in working form, but is maked WIP because
 - no tests are included
 - `installer` **doesn't seem like the right name for the stanza**.
     - we are changing `install` to `pkg`, so that would reduce one source of confusion.  But the Apple-provided CLI tool for installing `pkg` files is also called `installer`.
     - what is a better name?
         - `install_script` ?
         - `install_command` ?
         - `installer_script` ?
         - `installer_command` ?
         - `custom_installer` ?
         - `cli_installer` ?

References: #4688