Given a number, return a string which is formatted into US Dollars and cents!
Remember that:
dollaDollaBills(10) ➞ "$10.00"
dollaDollaBills(1000000) ➞ "$1,000,000.00"
dollaDollaBills(-314159.2653) ➞ "-$314,159.27"
dollaDollaBills(-56.99) ➞ "-$56.99"