Add convenience methods to remove action items from the button

[jjochen]

resolves #239

Adds two methods:

func removeItem(_ item: JJActionItem) -> JJActionItem?
func removeItem(at index: Int) -> JJActionItem

[codecov]

Codecov Report

Merging #240 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #240   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          10     10           
  Lines         995   1000    +5     
=====================================
+ Hits          995   1000    +5

Impacted Files	Coverage Δ
Sources/JJFloatingActionButton.swift	100% <100%> (ø)	⬆️

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