Add bounces property for disabling over-scroll bouncing.

[msaps]

• Add bounces property to PageboyViewController.
• Add updateContentOffsetForBounceIfNeeded function to update contentOffset for bounces preference.

Resolves #19

[codecov]

Codecov Report

Merging #20 into develop will increase coverage by 0.15%.
The diff coverage is 78.26%.

@@             Coverage Diff             @@
##           develop      #20      +/-   ##
===========================================
+ Coverage    73.11%   73.27%   +0.15%     
===========================================
  Files           13       13              
  Lines          729      752      +23     
===========================================
+ Hits           533      551      +18     
- Misses         196      201       +5

Impacted Files	Coverage Δ
Sources/Pageboy/PageboyViewController.swift	73.58% <ø> (ø)	✅
Sources/Pageboy/PageboyScrollDetection.swift	59.13% <71.42%> (+1%)	✅
Sources/PageboyTests/PageboyTransitionTests.swift	90.06% <88.88%> (-0.08%)	❌

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