FEATURE: Ability to specify a list of RTL locales

It would be nice to be able to do something like:

```ruby
I18n.available_locales = %i[en fr ar]
I18n.rtl_locales = %i[ar]
```

Then we could have a helper method for:

```ruby
I18n.rtl_locale?
```

I can make a PR, but wanted to check if there is appetite for something like this?