bugfix: make uninstall :script accept a hash

The indentation in CONTRIBUTING.md implies that :script accepts a hash.
But that is not the case. Instead :script accesses the entire hash defined
by uninstall.  Unrelated install keys such as :quit leak through to
@command.run! when attempting to exec :script.  Result: contrary to docs,
:script cannot be combined with other uninstall keys.

This PR makes uninstall :script accept a hash.  When :args or :input are not
needed, :script can still accept a plain string, so many Casks require no
alteration.

In addition
- adds key :executable, required when using hash argument to :script
- keys to :script are validated and sanitized before being passed to
  @command.run!
- adds :quit to with-installable.rb test to protect against regression
  on leaky keys
Merge pull request #2389 from rolandwalker/uninstall_script_hash

bugfix: make uninstall :script accept a hash