Formatting price

In this lesson, we're going to make it so the prices that we display on the screen are correctly formatted with two digits after the decimal point, and with a dollar sign as a prefix.

The reason I'm specifying two digits after the decimal point, is that, currently, that's not enforced. If you add lots of items to the cart, you'll see we occassionally get these crazy long numbers. [add multiple items to the cart and see the total increase]