make friction properly slow down the drag,

[micimize]

closes #53

Additionally, adds an AnimationPadding padding argument to the controller to allow for clamping the animation values. I'm honestly not 100% sure if it's the best solution for preventing the "fling off screen" issue, but it works fairly well with a MEDIUM stiffness:

      springDescription: SpringDescription.withDampingRatio(
        mass: 1,
        stiffness: Stiffness.MEDIUM,
        ratio: DampingRatio.LOW_BOUNCY,
      ),

[micimize]

@mcrovero Ok - added a AnimationPadding.none() option and made null topandbottomarguments default to+/- double.infinity`.

AnimationPadding.none() seems to result in package:flutter/src/widgets/basic.dart': Failed assertion: line 2300 pos 15: 'heightFactor == null || heightFactor >= 0.0': is not true. when flung to the bottom, so the more useful option is AnimationPadding(bottom: 0), which only pads the bottom of the screen.

[mcrovero]

Sorry for the late response, been overwhelmed lately. I think it's perfect!
I've just seen this error popping out in the logs when flinging the bottomsheet to the bottom

flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building AnimatedBuilder(animation:
flutter: RubberAnimationController#7b608(▶ -0.029), dirty, state: _AnimatedState#939e9):
flutter: 'package:flutter/src/widgets/basic.dart': Failed assertion: line 2310 pos 15: 'heightFactor == null
flutter: || heightFactor >= 0.0': is not true.
flutter:
flutter: The relevant error-causing widget was:
flutter: AnimatedBuilder
flutter: file:///Users/mattiacrovero/Desktop/Projects/Flutter/Rubber/rubber/lib/src/bottom_sheet.dart:175:14
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #2 new FractionallySizedBox (package:flutter/src/widgets/basic.dart:2310:15)
flutter: #3 RubberBottomSheetState._buildAnimatedBottomsheetWidget (package:rubber/src/bottom_sheet.dart:151:12)
flutter: #4 RubberBottomSheetState._buildSlideAnimation (package:rubber/src/bottom_sheet.dart:137:16)
flutter: #5 AnimatedBuilder.build (package:flutter/src/widgets/transitions.dart:1124:19)

[micimize]

closed in favor of #66