Now allow alpha in icons

[stephanecopin]

This is to allow support for alpha in MacOS icons, rather than have it being converted to black.

[codecov[bot]]

Codecov Report

Merging #20 (53eef58) into master (fd76815) will decrease coverage by 51.82%. The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master      #20       +/-   ##
===========================================
- Coverage   97.86%   46.04%   -51.83%     
===========================================
  Files          11       11               
  Lines         281      278        -3     
===========================================
- Hits          275      128      -147     
- Misses          6      150      +144     

Impacted Files	Coverage Δ
lib/ios_icon_generator/helpers/generate_icon.rb	12.50% <0.00%> (-85.75%)	:arrow_down:
lib/ios_icon_generator/helpers/mask_icon.rb	16.66% <0.00%> (-78.69%)	:arrow_down:
lib/ios_icon_generator/cli/runner.rb	20.68% <0.00%> (-72.65%)	:arrow_down:
lib/ios_icon_generator/helpers/which.rb	37.50% <0.00%> (-62.50%)	:arrow_down:
...os_icon_generator/helpers/image_sets_definition.rb	45.45% <0.00%> (-54.55%)	:arrow_down:
lib/ios_icon_generator/cli/commands/generate.rb	66.66% <0.00%> (-33.34%)	:arrow_down:
lib/ios_icon_generator/cli/commands/stub.rb	68.42% <0.00%> (-31.58%)	:arrow_down:
...b/ios_icon_generator/helpers/check_dependencies.rb	57.14% <0.00%> (-28.58%)	:arrow_down:
lib/ios_icon_generator/cli/commands/mask.rb	80.48% <0.00%> (-19.52%)	:arrow_down:
... and 1 more

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