add assertions isDivisibleByFive and containsFive

[chrkhl]

I added the following two assertions:

five.isDivisibleByFive();
five.containsFive();

Examples:

five.isDivisibleByFive(8); // false
five.isDivisibleByFive(15); // true
five.containsFive(154); // true
five.containsFive('this string contains 5, or not?'); // true
five.containsFive([ 2, 5, 17 ]); // true
five.containsFive([ 8, 14, 53 ]); // true

Maybe you find it somehow useful 😉