Add :rounded_infinite_precision option to .format

I think this option was missing, and it could be useful for money-rails `humanized_money` helper (I will maybe do a PR there when money-rails will use the new release of money)

```ruby
Money.infinite_precision = true
Money.new(BigDecimal.new('12.1'), "USD").format
#=> "$0.121"
Money.new(BigDecimal.new('12.1'), "USD").format(:rounded_infinite_precision => true)
#=> "$0.12"
```

Of course, it takes `subunit_to_unit` property in account.