Count the divisors of a number

Count the number of divisors of a positive integer n.

Random tests go up to n = 500000.

Examples

divisors(4)  == 3  //  1, 2, 4
divisors(5)  == 2  //  1, 5
divisors(12) == 6  //  1, 2, 3, 4, 6, 12
divisors(30) == 8  //  1, 2, 3, 5, 6, 10, 15, 30