Always merge I18n format values, namespaced or not, over the default
ones, to ensure I18n format defaults will have precedence over our
namespaced values.

Precedence should happen like this:

    default :format
    default :namespace :format
    i18n    :format
    i18n    :namespace :format

Because we cannot allow our namespaced default to override a I18n
:format config - ie precision in I18n :format should always have higher
precedence than our default precision for a particular :namespace.

Also simplify default format options logic.