Expose the presentation controller in TransitionContext (#26)

#25

Author: randcode-generator

src/MDMTransitionContext.h

@@ -79,4 +79,8 @@ NS_SWIFT_NAME(TransitionContext)
  */
 @property(nonatomic, strong, readonly, nonnull) UIView *containerView;
 
+/**
+ The presentation view controller for this transition.
+ */
+@property(nonatomic, strong, readonly, nullable) UIPresentationController *presentationController;
 @end

src/private/MDMViewControllerTransitionContext.m

@@ -20,13 +20,13 @@
 
 @implementation MDMViewControllerTransitionContext {
   id<UIViewControllerContextTransitioning> _transitionContext;
-  UIPresentationController *_presentationController;
 }
 
 @synthesize direction = _direction;
 @synthesize sourceViewController = _sourceViewController;
 @synthesize backViewController = _backViewController;
 @synthesize foreViewController = _foreViewController;
+@synthesize presentationController = _presentationController;
 
 - (nonnull instancetype)initWithTransition:(nonnull id<MDMTransition>)transition
                                  direction:(MDMTransitionDirection)direction