Recursion: Count Vowels

Write a function that recursively returns the number of vowels in a string.

If it wasn't clear enough already, you should use recursion in your solution.

Examples

vowels("apple") ➞ 2

vowels("cheesecake") ➞ 5

vowels("bbb") ➞ 0

vowels("") ➞ 0

Notes

• Recursive functions call themselves.
• All letters will be in lower case.
• For this challenge, the vowels are a, e, i, o, and u.