v0.7.0

• BREAKING: increases minimum SDK to 2.17
• BREAKING: changes default background color from ThemeData.backgroundColor to ThemeData.cardColor
• BREAKING: changes state type of ActionSliderController from SliderMode to ActionSliderControllerState
• adds anchorPosition and allowedInterval to ActionSliderController
• adds anchorPosition and allowedInterval to SliderState
• adds ActionSliderController.dual
• closes #6