Create translatable module

Avoid repeated code and makes way for a more abstract implementation
Create SpreeI18n::Config instance

It separates configs related to this extension from spree/core configs
Set Rails i18n.fallbacks true

I believe this is a safe default for spree_i18n since otherwise users
might see lists with empty names and forms not validating properly as
they should
Use symbols not strings on SpreeI18n::Config options