Consider adding a `filter` method to `Maybe`/`Either`

I've seen this in some other libs and it seems like a good idea.  The idea would be to have a method like `Maybe.filter` that would take a `callable` predicate and would return `Nothing` in the `Nothing` case and for the `Just` case would return the same `Just` value if the predicate returned true and `Nothing` if the predicate returns false.
