CowboySwagger should support either binaries or io_lists for descriptions

[HernanRivasAcosta]

Right now, it only accepts strings that it checks with io_lib:printable_list/1

[elbrujohalcon]

It's already working as good as it can possible work.
If anything, I would go exactly the other way: I would only accept binaries.
But as it is now, swagger understands both binaries and printable lists as json strings.
That's more than enough.