FFImageLoading 2.1.7

Complete history of changes available here: v2.1.6...v2.1.7

Fixes:

• Xamarin Forms UWP app crashes when quickly adding/removing CachedImage views
• IImageService fixes
• iOS TintTransformation fix
• Fixed #275 LoadingPlaceholder issues
• Added constructors to avoid missing ctr method exceptions

Enhancements:

• Added AsUIImageAsync and AsBitmapDrawable task extensions