Add an example of imperative-style if statement.

This code declares a variable, filename, and initializes it to
a default value. It then uses an if expression to check whether any
arguments were supplied to the program. If so, it changes the
variable to hold the value specified in the argument list. If no
arguments were supplied, it leaves the variable set to the default
value.

This code can be written more nicely, because `if` is an expression
in Scala that results in a value.
Rewrite the if expression in a more functional style.

This time, the if has two branches. If args is not empty, the
initial element, args(0), is chosen. Else, the default value is
chosen. The if expression results in the chosen value, and the
filename variable is initialized with that value. The code is
slightly shorter, but the real advantage is that it uses a val
instead of a var.

Using a val is the functional style, and it helps you in much the
same way as a final variable in Java. It tells readers of the code
that the variable will never change, saving them from scanning all
code in the variable's scope to see if it ever changes.
Add an example of a while expression.

The while loop in Scala behaves as in other languages. It has a
condition and a body, and the body is executed over and over as
long as the condition holds true.
Add an example of a do-while loop.

Exactly the same as in other languages, loop at least once before
testing the condition at the end of the loop.