Add Rubocop and checklist
Replace deprecated File.exists? method

Done with `rubocop --auto-correct --only DeprecatedClassMethods`
Replace deprecated Hash methods

Done with `rubocop --auto-correct --only DeprecatedHashMethods`
Clean up Array indenting

Done with `rubocop --auto-correct --only IndentArray`
Clean up Hash indents

Done with `rubocop --auto-correct --only IndentHash`
Clean up inconsistent identation

Done with `rubocop --auto-correct --only IndentationConsistency`
Clean up argument spacing

Done with `rubocop --auto-correct --only SingleSpaceBeforeFirstArg`
Fix spacing around block braces

Done with `rubocop --auto-correct --only SpaceBeforeBlockBraces`
Fix spacing before comments

Done with `rubocop --auto-correct --only SpaceBeforeComment`
Fix spacing inside block braces

Done with `rubocop --auto-correct --only SpaceInsideBlockBraces`
Fixed redundant use of #to_s in string interpolation

Done with `rubocop --auto-correct --only StringConversionInInterpolation`
Remove unused block arguments

Done `rubocop --auto-correct --only UnusedBlockArgument`
Update rubocop todo list