ITP - loops & iterators - countdown

A while loop is given a parameter that evaluates to a boolean (remember,
that's just true or false). Once that boolean expression becomes false,
the while loop is not executed again, and the program continues after
the while loop. Code within the while loop can contain any kind of logic
that you would like to perform. Let's try an example of a while loop by
creating a file named countdown.rb. We want this program to countdown
from any number given by the user and print to the screen each number as
it counts. Here we go!