feat(format-adapter): adapting asynchronously loaded content, from some custom format to JSON format.

I need this to be able to load ".properties" files without a server-side service.

Thanks
Please be sure that a custom standard function would also work. Similar change like https://github.com/angular-translate/angular-translate/pull/712/files#diff-255d6d49cbc024fa0d5bc97fdd721bf4R919