Add onPositionChange() or onBackDropPress() Method

[Aiiros]

Issue Description

This is a feature request is directly related to #96

When user taps on the backDrop, the slider is closed but no methods are triggered.
There is a cumbersome workaround: #96 (comment), but i think this should be a feature

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Environment

• Version: V2.3.1
• React Native version: 0.59.9
• Platform(s) (iOS, Android, or both?): Both

Edit: Duplicate of #127

[vjsingh]

Seconded. Would be great if there was a callback whenever the background changed to a collapsed or open state.