Add support for introsort

[boristane]

Sorts an array using the introsort algorithm.

const arr = [2, 5, 10, 5, 32, 6];
intro(arr); // [2, 5, 5, 6, 10, 32]
intro(arr, (a, b) => b - a); // [32, 10, 6, 5, 5, 2]

Reference

Update README.md.