Add `default` parameter for missing scopes

Hi,

The library currently provides an option for providing `defaultValue` if the provided scope is missing.

Rails provides the ability to specify a default scope in addition to a default value:
`I18n.t :missing, default: [:also_missing, 'Not here']` (http://guides.rubyonrails.org/i18n.html#bulk-and-namespace-lookup)

Would you be interested in a pull request for this?