Please add ribimaybe, a Maybe Functor, Applicative and Monad.

`ribimaybe` is a tiny Ruby library that provides a Maybe datatype which is a Functor, Applicative Functor and Monad instance. The entire project directory contains `193` lines of code (the main module itself contains `66`) and I hope for it to be used in production very soon.

**URL:** https://github.com/unsymbol/ribimaybe