A prime number is a number that is only evenly divisble by itself and 1.
For example, the number 5 is prime because it can only be evenlly divided by 1
and 5. The number 6, however, is not prime because it can be divided evenly
by 2 and 3.
Write a Boolean function named is_prime which takes an integer as an argument
and returns true if the argument is a prime number, or false otherwise. Use
the function in a program that prompts the user to enter a number and then
prints whether the number is prime.