Allow binding a signal other than command.enabled to the control's enabled

Using NSControl+RACCommandSupport, it's not currently possible to set up a connection to an object such that it has access to the control's command signal in its initialization while the control's `enabled` is bound to a signal coming from that same object:
    
    [aButton setRac_command:
        [[RACCommand alloc] initWithSignalBlock:^RACSignal *(NSButton * _) {
                                        return [RACSignal return:@YES];
    }]];
    
    RACSignal * buttonSignal = [[aButton rac_command] executionSignals]; 
    
    MyController * controller = [[MyController alloc] initWithButtonSignal:buttonSignal];

    // Does not exist
    [aButton <#bind enabled to#>:[controller aSignal]];

The alternative

    // Set up signal as best as possible without button
    MyController * controller = [MyController new];
    
    [aButton setRac_command:
        [[RACCommand alloc] initWithEnabled:[controller aSignal]
                                signalBlock:^RACSignal *(NSButton * _) {
                                        return [RACSignal return:@YES];
    }]];

    [controller setButtonSignal:[[aButton rac_command] executionSignals]];

necessitates a potentially awkward (certainly repetitive) re-wiring of the controller's already-created signal in `setButtonSignal:`

I propose adding a single method to `NSControl+RACCommandSupport` to allow the control's `enabled` to be rebound to any signal after the command is set. This pull request implements that solution, and includes a test for the new functionality.

The change is very minor; the existing code binding the control's `enabled` in `setRac_command:` was simply moved to its own method.

If you think it's necessary that the command's `enabled` be involved in the control's status, my alternate proposal is to allow setting a "secondary" signal whose values will be ANDed with `command.enabled`. I have that code in a branch NSControl-mergeEnabledBindingSignals in my fork of RAC and can submit another pull request.