Centered Messages

• Added .center presentation style with a physics-based dismissal gesture.
• Added .custom(animator:) presentation style, where you provide an instance of the Animator protocol. The TopBottomAnimation and CenterAnimation animations both implement Animator and may be subclassed (configuration options will be added in a future release). You may use the PhysicsPanHandler class to incorporate a physics-based dismissal gesture into any custom animator.
• Added .centered message view layout with elements centered and arranged vertically.
• Added configureBackgroundView(width:) and configureBackgroundView(sideMargin:) convenience methods to MessageView.