Add configuration option that makes the default when a translation file is missing a string be to fall back to the string from the defaultLang

[danielhjacobs]

#41 as a configuration option.

We use Crowdin for all our translations and in Crowdin the default is to leave translation files blank until users provide translations. As it stands, we need to either not add a language to the navigation bar drop-down until every string has been translated or fill in all the languages with English at first which is an anti-pattern.

[martinkr]

Hi @danielhjacobs,

Thank you for your contribution.
Would you consider opening a PR for your idea? I would happily merge it and release a new version if you do that :)

Thank you!