Multi fizz

Very similar to the previous FizzBuzz. Changes:
- ability to read a file of settings (pairs multiple, string)
- instance variable @settings stores those values as a hash
- we order the keys in ascending order and select those that are modulus
- extract the way we present the string to another method because the clients want to be able to decide later how to present the results. This way is easier to maintain.
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.