Return both raw and rounded values

While calculating [overall-loan-cost](https://github.com/cfpb/overall-loan-cost) values, I noticed that they were occasionally off by a cent. That module uses amortize as a dependency to calculate the loan amortization table. It's like that these nice round values caused the slight miscalculations.

Now the module returns the following:

```javascript
{ 
  interest: 36583.362108097754,
  principal: 16546.146128485594,
  balance: 163453.85387151438,
  payment: 885.4918039430557,
  interestRound: '36583.36',
  principalRound: '16546.15',
  balanceRound: '163453.85',
  paymentRound: '885.49'
}
```