9.16 lab: vending machine
given two integers as user inputs that represent the number of drinks to buy and the number of bottles to restock, create a vendingmachine object that performs the following operations:

purchases input number of drinks
restocks input number of bottles
reports inventory
a vendingmachine's initial inventory is 20 drinks.

ex: if the input is:

5
2
the output is:

inventory: 17 bottles