Expose _contentViewController & _menuViewController to subclass

This PR enables subclass creating its own initialization method to init `contentViewController` & `menuViewController` like `-initWithContentViewController:menuViewController:` method does: Use `_XXXViewController` instead of `self.XXXViewController`.

e.g.

We can use own RESideMenu subclass with content & menu view controllers wrapped, and just use `-init` method to do initialisation job (in my case, I can only use `-init` method for some reason):

```Obj-C
- (id)init
{
  if (self = [super init]) {
    _sampleContentViewController = [[SampleContentViewController alloc] init];
    _sampleNavigationController = [SampleNavigationController alloc];
    (void)[_sampleNavigationController initWithRootViewController:_sampleContentViewController];
    
    _sampleMenuViewController = [[SampleMenuViewController alloc] init];
        
     _contentViewController = _checkoutNavigationController;
     _menuViewController    = _sidebarMenuViewController;
  }
  return self;
}
```