feat(iExpense): add challenges

[plskz]

Challenges

1. Use the user’s preferred currency, rather than always using US dollars.
2. Modify the expense amounts in ContentView to contain some styling depending on their value – expenses under $10 should have one style, expenses under $100 another, and expenses over $100 a third style. What those styles are depend on you.
3. For a bigger challenge, try splitting the expenses list into two sections: one for personal expenses, and one for business expenses. This is tricky for a few reasons, not least because it means being careful about how items are deleted!

Screenshot