Updated #69 - Add exists? to check if a certain key exists in the translations

Implemented [Jose Valim's suggestions](https://github.com/svenfuchs/i18n/pull/69#issuecomment-553998) for #69:

* Moved `exists?` to `I18n::Backend::Base`
* Added implementation for `I18n::Backend::Chain`
* Added tests to `I18n::Tests::Basic`