Provide a custom authorization action on parent resources

[phallguy]

Currently, when authorizing nested resources, the parent resource is always authorized with the :show action. This makes a great deal of sense and is a good default but there are occasions when are more restrictive option is needed for more complex permissions.

I'd like to be able to specify the action that cancan uses to authorize the parent object as well

class Controller
  load_and_authorize_resource :library, parent: true, parent_action: :sort
end

[Senjai]

@phallguy I know it's been a while, but I think this is a fair suggestion. Would you consider submitting a pull request?

[phallguy]

Sure thing see #216