Add interactive sliding menu as an example

[randcode-generator]

https://github.com/material-motion/transitioning-objc/issues/20

[codecov[bot]]

Codecov Report

Merging #22 into develop will decrease coverage by 12.26%. The diff coverage is 31.25%.

@@             Coverage Diff              @@
##           develop      #22       +/-   ##
============================================
- Coverage    80.13%   67.87%   -12.27%     
============================================
  Files            3        3               
  Lines          146      193       +47     
============================================
+ Hits           117      131       +14     
- Misses          29       62       +33

Impacted Files	Coverage Δ
src/UIViewController+TransitionController.m	76% <0%> (-24%)	:arrow_down:
src/private/MDMPresentationTransitionController.m	64.47% <34.61%> (-15.53%)	:arrow_down:
src/private/MDMViewControllerTransitionContext.m	68.47% <37.5%> (-6.85%)	:arrow_down:

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