support optional languages.

Hi

I added ability to specify loading locales. It can be useful in cases when I'm using not only available locales in my app. 

For example:
```ruby
require "i18n/backend/fallbacks"
I18n::Backend::Simple.send(:include, I18n::Backend::Fallbacks)
I18n.fallbacks.map(zh_CN: :zh)

config.i18n.available_locales = [:en, :zh_CN]
config.i18n_countries_translations.locales = [:en, :zh]  # <-- new option
```
and I can still use your gem to provide countries names from zh locale inspite of I haven't :zh locale in my i18n.
```ruby
I18n.t(:US, scope: :countries, locale: :zh_CN) # => "美国"
I18n.t(:US, scope: :countries, locale: :zh)  # => I18n::InvalidLocale: :zh is not a valid locale
```