Given list of strings and a string return index of th value in the list or -1 if value was not found.
Don't use any Ruby build-in methods that are directly returning index of element like index / find_index, etc.
get_index(%(A B C), 'A') # 0
get_index(%(A B C), 'B') # 1
get_index(%(A B C), 'D') # -1