Handle the combinatory teen cases
Add examples for the twenties
Add example for twenty
Rename Digit#word to single word, and remove paramenter
Add sample for the rest of combinatory teens
Refactor: Use ternary
Add alias for clarity of intentions
Add examples for the hundreds
Add example for hundreds with zero in the middle
Extract classes to separated files
Digit::new use keyword arguments
Concat digit conversions using join
Add parameter nil_zero to SingleDigit#to_words