Allow for declarative tween defaults using an `InheritedWidget`

In some cases you want a group of tweens to share some default duration, curve, or delay. It might be nice if this could be declared in the widget tree:
```dart
return AnimateDefaults(
duration: 1.seconds,
delay: .5.seconds,
curve: Curves.easeInOut,
child: Column(
children: [
childA.animate().fadeIn();
childB.animate().fadeIn().scale(...);
childC.animate().fadeIn().slide(...);
]
)
);
```

These would simply act as new defaults, nothing more fancy than that.

This could be accomplished by with `class AnimateDefaults extends InheritedWidget`, and then calling `AnimateDefaults.maybeOf(context)` from within `Animate` to look up the closest ancestor with default overrides (if any).