[Suggestion] Add calculation by monthly interest and add months installments

[rafaell-lycan]

Hey @kfiku, in Brazil loans are calculated by months, both installments and the interest.

Is it possible to adapt as an option installments year and month? Like 18 months installmentYear: 1 and installmentMonth: 6, and then, add an option to use the interest based on monthly or annual.

I really liked the lib, I'll try to figure out this week how to adapt and make a pull request.