Added support for handling a variety of user input

The program can now handle when the user inputs a dollar sign, commas, cents, etc.

Unfortunately, this does not support Europeans yet.

Here's some small code I used to test the variety of inputs:
```
goal_dollars_combinations = ["1000000", "1,000,000", "1,000,000.00", "$1,000,000.00"]
for goal_dollars in goal_dollars_combinations:
    # Strip out any dollar signs
    goal_dollars = goal_dollars.replace('$', '')
    # Strip out any commas before converting to an int, so that an input like "1,000" is handled properly
    goal_dollars = goal_dollars.replace(',', '')
    # Strip out anything after a period, since we're not taking cents into account (allowing us to handle "100.00")
    goal_dollars = goal_dollars.partition('.')[0]
    # TODO: Support Europeans
    goal_dollars = int(goal_dollars)
    print(str(goal_dollars))
```