Add sketch of I18n error translation

Super simple possible implementation of I18n support for Liquid errors, as requested in #18. I haven't done all the actual replacing yet. 

The only performance impact this has is read the YAML file into memory to have the errors handy. Even that isn't necessary though, as long as information is stored about what localization is needed, then the `I18n` object could be created ad-hoc.

@fw42 please take a look.