use .i18nrc

[jenseng]

since i18nliner and i18nliner-js can both use the same .i18nignore file, it might also be nice to share the .i18nrc file

this way you could easily set things like inferred_key_format, etc. in one place. see this commit for context. we don't need to worry about plugins, since in ruby land adding a gem should suffice