DSL: add `install_script` stanza

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 almost working form.  It is marked WIP because~~
 - ~~tests are failing~~ FIXED?
 - ~~`installer` doesn't seem like the right name for the stanza~~ 
     - **Settled on `install_script` for consistency with `uninstall :script`**
     - `install :script` would be even more consistent, but the overlap with an existing stanza name would make a transition difficult
     - ~~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`  <----- consistent with `uninstall :script`**
         - ~~`install_command` ?~~
         - ~~`installer_script` ?~~
         - ~~`installer_command` ?~~
         - ~~`custom_installer` ?~~
         - ~~`cli_installer` ?~~

References: #4688