Add validations for options

Hey folks! 🖖

Thanks for the excellent library, httpoison is awesome.

I ran into an issue where all my requests were failing with the `checkout_failure` error reason. After some debugging, I found that the `timeout` option I was passing to httpoison was accidentally set to a stringfied integer instead of an integer.

So I had the idea of adding some sort of options validation to HTTPoison. The validation would make the error I had much easier to see since it is not obvious that a string will be interpreted as `0`.

I think the easiest approach to implement it would be to use [dashbitco/nimble_options](https://github.com/dashbitco/nimble_options).

This would obviously be an enhancement and not a bugfix, but IMO it would be nice to have because it will hopefully save developers time with debugging.