Plus the count of [Iterator::enumerate] by base

Example

use crate::enumerate_base::IteratorEnumerateBaseExt as _;
let arr = ['a', 'b', 'c', 'd'];
let iter = arr.into_iter().enumerate().enumerate_base(2);

assert_eq!(iter.collect::<Vec<_>>(), vec![
    (2, 'a'),
    (3, 'b'),
    (4, 'c'),
    (5, 'd'),
]);