Given an initial amount, %interest/day, and the number of days, will
compute the new total amount given compound interest.
Usage:
$ loan_calc.pl <$initial> <%interest/day> <days>
Example:
$ loan_calc.pl 1.00 .06 7
7 days later...
Given a loan of $1.00 and a daily interest rate of 6%,
$1.50 will be owed ($0.50 earned/lost)