[MDCSwipeOptions] exitScreenOnChosenWithDuration - configurable

[grabbou]

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Hi, It'd be quite cool to have an ability to set that duration instead of having predefined 0.1 or overriding entire onChosen property. Can do PR for that by introducing another static property to header file e.g.

@property (nonatomic, assign) NSTimeInterval exitScreenAnimationDuration;

instead of

options.onChosen = [MDCSwipeOptions exitScreenOnChosenWithDuration:0.5 options:UIViewAnimationOptionCurveLinear];

What do you think?