Ability to query cities by country name in addition to country code

[sarmad90]

Hey! I'm using this gem, going well so far. I needed to query cities with country name as compared to only being able to query cities with country code, so I've written a small method to do that. Take a look if you find it useful and think it should be merged. Thanks.

[TahaMaqbool]

@sarmad90 I did this to solve this problem
CS.states(:us).keys.flat_map { |state| CS.cities(state, :us) }